Symmetry, usually make a vertical line after crossing the point.

1, set the vertical line passing through the point as x-y-k =0.

Introduction point (-3,4) k =-7.

The vertical line and the straight line are connected to find the intersection point (-2.5, 4.5).

This point is the midpoint. Symmetry point x = 2 *(2.5)-(-3)=-2,

y= 2*(4.5) - 4 =5

That's it.

The answer is C.