[tex]P = 60\\\\\frac{1}{2}xy = 60 \ \ /\cdot2\\\\xy = 120\\\\y = x+7\\\\x(x+7) = 120\\\\x^{2}+7x-120 = 0\\\\a = 1, \ b = 7, \ c = -120\\\\\Delta = b^{2}-4ac = 7^{2}-4\cdot1\cdot(-120) = 49 + 480 = 529\\\\\sqrt{\Delta} = \sqrt{529} = 23\\\\x_1 = \frac{-b-\sqrt{\Delta}}{2a} = \frac{-7-23}{2\cdot1} = \frac{-30}{2} = -15 \ \notin D\\\\x_2 = \frac{-b+\sqrt{\Delta}}{2a} = \frac{-7+23}{2} = \frac{16}{2} = 8\\\\x = 8[/tex]