**Enforceability of the Agreement**

The enforceability of the agreement between Mr. Arhan and Mr. Farhan depends on whether it is legally valid and enforceable under the principles of contract law. To determine this, we need to consider the following elements:

**1. Agreement between the Parties**
There must be a valid agreement between the parties involved. In this case, Mr. Arhan and Mr. Farhan have agreed that Mr. Arhan will not practice the same trade for 3 years and will not carry on any business competing with Mr. Farhan's business. This agreement is likely to be in the form of a written contract, which is a legally recognized form of agreement.

**2. Consideration**
For a contract to be valid, there must be consideration exchanged between the parties. Consideration refers to something of value that is exchanged between the parties to make the agreement legally binding. In this case, the consideration is the sale of goodwill from Mr. Arhan to Mr. Farhan. As long as there is valid consideration, this element is satisfied.

**3. Legality of Object**
The object or purpose of the agreement must be legal. In this case, the agreement restricts Mr. Arhan from practicing the same trade and competing with Mr. Farhan's business for a certain period of time. Such non-compete clauses are generally considered legal and enforceable, provided they are reasonable in terms of duration, geographic scope, and industry.

**4. Capacity of the Parties**
To enter into a legally binding agreement, the parties must have the legal capacity to do so. This means they must be of sound mind and of legal age. Assuming that Mr. Arhan and Mr. Farhan meet these requirements, this element is satisfied.

**Enforceability**
Based on the above analysis, it can be concluded that the agreement between Mr. Arhan and Mr. Farhan is likely to be enforceable. The agreement meets the essential elements of a valid contract, including agreement, consideration, legality of object, and capacity of the parties. However, it is important to note that the enforceability of non-compete clauses may vary depending on the jurisdiction and specific circumstances of the case. Therefore, it is advisable to seek legal advice to determine the enforceability of such clauses in a particular jurisdiction.