3x + 12 + x = 4x + 12 becuase 3x and x are like terms so you can add them
4(3 + x) = 12 + 4x because 4 times 3 is 12 and 4 times x is 4x so you add them to get 12 + 4x which can be rearranged to equal 4x + 12
So in short: 3x + 12 +x is equivalent to 4(3+x)
