Both given points have the same Y coordinate, so this can be the base of the triangle. The length of the base id the sum of the X values:
-3 to 3 is 5 units long.
Area of a triangle is 1/2 x base x height.
The area is given as 20 square units.
Calculate the height:
20 = 1/2 x 5 x Height
20 = 2.5 x height
Height = 20/2.5
Height = 8 units.
Using the point (-2,-3) keep the x value -2 then add 8 units to the Y value for the height:
The 3rd vertex could be (-2,5)