M = (x₁ + x₂)/2 , (y₁ + y₂)/2
(-1, 2) = [tex]\frac{3 + x}{2} , \frac{4 + x}{2}[/tex]
At this point, it is easier to separate the x's and y's into two different equations:
x's: -1 = [tex]\frac{3 + x}{2}[/tex]
-2 = 3 + x
-5 = x
y's: 2 = [tex]\frac{4 + x}{2}[/tex]
4 = 4 + x
0 = x
Answer: (-5, 0)