ANSWER
48
EXPLANATION
If two polygons are similar, then the ratio between the corresponding sides is constant. This constant is known as the scale factor.
In this case, the ratios between corresponding sides are,
[tex]\frac{18}{15}=\frac{?}{40}[/tex]
Solving for the missing length,
[tex]?=\frac{18}{15}\cdot40=\frac{6}{5}\cdot40=48[/tex]
Hence, the missing length is 48.
