Answer:
The side lengths are 30, 36 and 39.
Explanation:
To find the missing side, write a proportion using side lengths from each similar shape. A proportion is an equation that sets equal two ratios.

Find y by cross multiplying numerator and denominator of each fraction.
13y = 39(12)
13y = 468
y = 36
Find the other missing side by repeating the process.

Find y by cross multiplying numerator and denominator of each fraction.
13y = 39(10)
13y = 390
y = 30
The side lengths are 30, 36 and 39.