Question 1117720
#29, triangle PQR
{{{QR = PQ+6}}}
-
{{{PR=QR+PQ-18}}}
-
{{{PR+PQ+QR=78}}}


Three equations in three unknown quantities.
Question asks for QR.  If you work with the set of equations carefully, you should be able to write the perimeter equation all in terms of QR.