I need to find the measure of side $YZ$ which is part of the similar triangle $XYZ$. Since the triangles are similar, that means their corresponding sides are proportional. I can write a proportion relating the sides of the two triangles: $\frac{AB}{XY}=\frac{BC}{YZ}$. I can plug in the given values of $AB, BC$ and $XY$: $\frac{4}{14}=\frac{6}{YZ}$. I can cross-multiply and solve for $YZ$: $4YZ=84$, so $YZ=21$. 
Triangle $ABC$ is similar to triangle $XYZ$ with side $AB$ measuring 4 units, side $BC$ measuring 6 units and side $XY$ measuring 14 units. What is the measure of side $YZ?$