You first subtract 5x from both sides and end up with -3y=21-5x. In order to isolate the variable you need to divide -3 from both sides. Your equation would then be y= -7+ 5/3x.
First set up the equation (Area)80=bh/2 then set up another equation, (height) h=4+2b (base). After this you can substitute h in the equation to end up with 80=(b(2b+4))/2 simplify it to get 80=b^2+2b then solve. The base is 8 meters, plug into the formula that we made before and you find the height is 20 meters.