We can use the fact that ADC = 112 to form and solve an equation.
12x - 3 + 6x + 7 = 112
18x - 3 + 7 = 112 ( collect the x values)
18x + 4 = 112
18x = 108 ( subtract both sides by 4 to get the x values alone)
x = 6 ( divide both sides by 18 to get what x is)
