9x - 2a - 2b = 100
9x - 2(a+b) = 100
9*19 - 2(a+b) = 100
171 - 2(a+b) = 100
171 - 100 = 2(a + b)
71 = 2(a+b)
a + b = 71: 2
a + b = 35,5
