I need help with geometry

Solve 2x+y=3 x+y=5 in substitution method

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

Erik buys 2.5 pounds of cashews. if each pound of cashews costs $7.70, how much will he pay for the cashews?