The midpoint is the average of the endpoint coords. So C is (-4,1).
The length of AC is √((-4-(-3))²+(1-(-4))²)=√(1+25)=√26=5.1 approx.
Another way is to find the length of AB then halve it:
AB=√((-5-(-3))²+(6-(-4))²)=√(4+100)=√104=2√26. Halve it: √26.