If L and W are length and. width perimeter=80=2(L+W) and area=375=LW.
So, 2L+2W=80, L+W=40 and W=40-L. Substitute for W in the area equation: L(40-L)=375.
40L-L^2=375, L^2-40L+375=0=(L-15)(L-25) so L=15 or 25 and W=25 or 15 so the dimensions are 15 by 25.