(6x7) = 47
(6x-7) = -42 -- This is because you are multiplying two differently signed numbers, the product will be negative.
Remember that when you bring ÷ to the other side of the equal sign it'll become multiply instead
2L + 2W = 36
2(3+2W) + 2W = 36
6 + 4W + 2W = 36
6W + 6 = 36
-6 = -6
6W=30
W= 30/6 = 5 L= 3 + 2W = 3 +2 (5) = 3 +10= 13
Width is 5
Length is 13