we will assume that x is a real number

3 + xi = (x+1) + 2i

3 = x + 1 (the real part on the left side is equal to the real part of the right side)

x + 1 = 3

x = 3 - 1

x = 2

x = 2 (the imaginary part of the left side is equal to the imaginary part of the right side)