Distribute 
to every term inside the parentheses.
Simplify with multiplication.
You can't simplify further by adding, so leave the answer as-is.
When two angles are supplementary, that means that the sum of their individual angles is equal to a straight line, which is 180°. If you add another restriction where the two supplementary angles are equal, then each vale of the angle constitutes half of 180°. Then, that means that ∠A = ∠B = 90°. The answer is C.
For this problem you have to isolate the variable. To do this you must inverse the operations. Multiply 18 by 5, then divide by 12 to get your answer of 7.5
