The length of one side is an unknown value subtracted from 60 and multiplied by 4.

\(\displaystyle P = 4(\,side\,)\)

\(\displaystyle P = 4(60 - x)\)

\(\displaystyle P = (240 - 4x)\)

\(\displaystyle -240 = -4x\)

\(\displaystyle p = 60\)

Check

\(\displaystyle P = (240 - 4x)\)

\(\displaystyle -240 = -4(60)\)

Yes.

Next

\(\displaystyle P = 4(60 - x)\)

\(\displaystyle 0 = 4(60 - 60)\)

Yes