Help me plz

Rewrite so the subject can be y in the formula d=m-y/y+n

Marquise has 200 meters of fencing to build a rectangular garden. the garden's area (in square meters) as a function of the garden's width w w (in meters) is modeled by: a ( w ) = − w 2 + 1 0 0 w a(w)=−w 2 +100w what side width will produce the maximum garden area? meters

Which shows how to solve the equation 3/4x=6 for x in one step?