AB + BC = AC
Replacing with the values:
3x + (5x + 10) = 74
(3x + 5x) + 10 = 74
8x + 10 = 74
10 is adding on the left, then it will subtract on the right
8x = 74 - 10
8x = 64
8 is multiplying on the left, then it will divide on the right
x = 64/8
x = 8