Here's the question you clicked on:
Godsgirl
What is the length of side BC? A: 6 B: 21 C: 9 D: 15
\[a^{2}+b ^{2}=c ^{2}\] \[15^{2}+\left( x+6 \right)^{2}=x ^{2}\] use foil
those lines there mean that the two sides are congruent and x would be 15 and 15 plus 6 is 21 so the answer is 21.