Find the other endpoint given the first endpoint and the midpoint A = (3, 5) B = (5, -11) find C

You could use the rise over run method

Find the difference between the first endpoint and midpoint x values which in this case would be +2 because(5-3=2)

Then do the same for their y values(-11-6)=?

After that add the difference in x values to the the midpoint x value(5+2)=
And the same for its y value(-11+?)=?

That will give you your second endpoint