midpoint means 1/2 way
separate the x's and the y's find the middle of the x length, middle of the y length
x: 5 - 3 = 2 1/2 of 2 = 1 from either end (3 + 1) or (5-1) = 4
y: 3-(-5) = 8 1/2 of 8 = 4 from either end (3-4) or (-5 +4) = -1
midpoint is therefore (4, -1)