SAT prep question - edit (with picture)

In the figure below, point QQQ lies on \overline{PR}​PR​​​start overline, P, R, end overline.

If the length of \overline{PR}​PR​​​start overline, P, R, end overline is 181818 and the length of \overline{QR}​QR​​​start overline, Q, R, end overline is twice the length of \overline{PQ}​PQ​​​start overline, P, Q, end overline, what is the length of \overline{PQ}​PQ​​​start overline, P, Q, end overline?