To find the coefficient of x in the expansion of (1 + x)^m(1 - x)^n, we can use the binomial theorem. According to the binomial theorem, the general term in the expansion of (a + b)^n is given by:

C(n, r) * a^(n-r) * b^r

where C(n, r) represents the number of ways to choose r items from a set of n items, and r ranges from 0 to n.

In this case, we have (1 + x)^m(1 - x)^n. To find the coefficient of x, we need to consider terms that contain x and its powers.

The coefficient of x will be obtained by multiplying the terms from (1 + x)^m and (1 - x)^n that contain x. Since we are given that the coefficient of x is 3, we can set up the equation:

C(m, 1) * 1^(m-1) * x^1 * C(n, 0) * 1^n * (-x)^0 = 3

Simplifying this equation gives us:

m * x * 1 = 3

Therefore, we have m = 3/x.

Similarly, to find the coefficient of x^2, we can set up the equation:

C(m, 0) * 1^m * x^0 * C(n, 2) * 1^(n-2) * (-x)^2 = -6

Simplifying this equation gives us:

n(n-1)/2 * x^2 = -6

Therefore, we have n(n-1) = -12/x^2.

Now, we can solve these two equations simultaneously to find the value of m.

From the first equation, we know that m = 3/x. Substituting this into the second equation gives us:

(3/x)(3/x-1) = -12/x^2

Simplifying this equation gives us:

9/x^2 - 3/x = -12/x^2

Multiplying through by x^2 gives us:

9 - 3x = -12

Rearranging this equation gives us:

3x = 21

Therefore, we have x = 7.

Substituting this value back into the first equation gives us:

m = 3/7

Since the value of m is not an integer, we can conclude that the correct answer is option C) 12.