When solving s system of 2 linear equations algebraically, how can you tel if the system has EXACTLY 1 Solution?

Which one ? is the answer and how to find this answer

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

Use the distance formula to find the distance between (-3,5) and (3,1)