(x1 + x2)/2, (y1 + y2)/2 is the formula for midpoint. Since you have the midpoint and one end point set it up like:
(-3 + x2)/2 = -1
(-3 + x2) = -2 (multiply both sides by 2
x2 = 1 (add 3 to both sides.)
(-1 + y2) / 2 = 1
(-1 + y2) = 2 (multiply both sides by 2
y2 = 3 (add 1 to both sides).
Your endpoint therefor is (1, 3)