A) Both terms contain x, so they can be combined. -25 + 36 = 11 therefore -25x + 36x = 11x
B) 7s and 3s both contain s, so they can be combined, but the 3 cannot be combined with them as it does not contain an s. 7 - 3 = 4 therefore 7s - 3s = 4s. The 3 stays on its own as it cannot be combined, therefore the answer is 4s + 3
C) 12x and 2x contain x, so they can be combined, but the 11 cannot be combined with them as it does not contain an x. 12 - 2 = 10 therefore 12x - 2x = 10x. The 11 stays on its own as it cannot be combined, therefore the answer is 10x - 11
