The new perimeter == = L*(1+0.25) + W*(1+0.25) + L*(1+0.25) + W*(1+0.25) = (1+0.25)*(L + W + L + W) = 1.25*(24 + 18 + 24 + 18) = = = 1.25*84 = 105 inches. ANSWER
Under this similarity enlarging, the new perimeter to the old perimeter ratio is the same as the ratio of the new side length to the old side length