#### Answer

x = 12

#### Work Step by Step

If any two triangles are similar, then their corresponding sides are in proportion.
Thus, the proportion of the given sides:
$\frac{8}{x}$ = $\frac{10}{15}$
Cross multiplying both sides, we get
15(8) = 10(x).
Thus 120 = 10x.
Dividing both sides by 10, we get
$\frac{10x}{10}$ = $\frac{120}{10}$.
Thus, x = 12.