The wavy line ~ means the triangles are similar. You can use ratio to solve for the side. If you rotate ΔXYZ counter-clockwise you can see that side SR=20 is side YZ=8 on the smaller triangle. This means that the side we are solving for QR matches up with side XZ=10 on the smaller triangle.
ratio : (I'll just use an X to represent side QR)
8/20= 10/X
cross multiply
8X = 200
X = 200/8
X = 25
