Test 5 Section 2 - #15 (page 642)

As with any geometry question, your first step is to fill in EVERYTHING YOU KNOW.

Because angles A and C are equal, AB = BC. So ABC has sides of 5, 8 and 8, and therefore a perimeter of 21.

Because y = 60, the third, unmarked angle in DEF must also be 60°, so DEF is equilateral. That means its sides are all 5, and its perimeter is 15.

21-15 = 6, so (C) is the answer.