Given:
The perimeter of the triangle, P=91.
The sides of the triangle are,
PR=4z
QR=z+3
PQ=5z-2.
The perimeter of the triangle can be expressed as,
[tex]\begin{gathered} P=PR+QR+PQ \\ P=4z+z+3+5z-2 \\ P=10z+1 \end{gathered}[/tex]Now, put P=91 in the above equation to find the value of z.
[tex]\begin{gathered} 91=10z+1 \\ 91-1=10z \\ 90=10z \\ \frac{90}{10}=z \\ 9=z \end{gathered}[/tex]Now, the length of the side QR can be calculated as,
[tex]\begin{gathered} QR=z+3 \\ QR=9+3 \\ QR=12 \end{gathered}[/tex]Now, the length of QR is 12 units.