A = l * w (definition)

l = w + 8 (define length)

A = (w + 8) * w (substitute l with w+8)

105 = (w + 8) * w (substitute A with 105)

105 = w * w + 8w (multiply both terms w+8 with w)

w * 2 + 8 * w - 105 = 0 (move 105 to other side)

Now you can use the a b c formula (a = 1, b = 8 c = -105) to find the solutions to w

You will find the answers -15 and 7. -15 is not valid, since with has to be larger then 0.

w = 7. (abc-formula)

l = w + 8 = 7 + 8 = 15 (substitute w with 7)

l = 15, w = 7.

15 * 7 = 105 (check the answer)

l = w + 8 (define length)

A = (w + 8) * w (substitute l with w+8)

105 = (w + 8) * w (substitute A with 105)

105 = w * w + 8w (multiply both terms w+8 with w)

w * 2 + 8 * w - 105 = 0 (move 105 to other side)

Now you can use the a b c formula (a = 1, b = 8 c = -105) to find the solutions to w

You will find the answers -15 and 7. -15 is not valid, since with has to be larger then 0.

w = 7. (abc-formula)

l = w + 8 = 7 + 8 = 15 (substitute w with 7)

l = 15, w = 7.

15 * 7 = 105 (check the answer)