Question

△abc and △def are similar. find the missing side length.
(the triangles are not drawn to scale.)

Explanation:

Step1: Find similarity ratio

$\text{Ratio} = \frac{BC}{EF} = \frac{15}{5} = 3$

Step2: Relate sides of similar triangles

Let missing side $AB = x$.
$\frac{AB}{DE} = 3$

Step3: Solve for missing side

$x = 3 \times DE = 3 \times 2 = 6$

Answer:

6