Since the number of terms is finite, just differentiate term by term using the power rule.
dy/dx = 0 + 1 + 2x/2! + 3x2/3! + ... + nxn-1/n!
dy/dx = 1 + x + x2/2! + ... + xn-1/(n -1)!
Compare dy/dx to y and note that the last term of y, which is xn/n!, is not in dy/dx, while all the other terms of y are in dy/dx.
Then dy/dx = y - xn/n!
⇒ dy/dx + xn/n! = y