Explanation:
the 4th point (vertex) must have the same offset from (-7, -3) as (3, -4) has from (-4, 2).
so, from (-4, 2) to (3, -4)
x has to increase by +7 (from -4 to 3).
y has to decrease by -6 (from 2 to -4).
calculating the same offset from (-7, -3) :
(-7 + 7, -3 - 6) = (0, -9)
so, the 4th vertex is at (0, -9).