The distance formula is: d = sqrt( (x2 - x1)2 + (y2 - y1)2 )
For this problem, let (-5, -4) be the "first" point, so x1 = -5 and y2 = -4
and let (-6, 4) be the "second" point, so x2 = -6 and y2 = 4.
Then: d = sqrt( (-6 - -5)2 + (4 - -4)2 ) = sqrt( (-1)2 + (8)2 ) = sqrt( 1 + 64 ) = sqrt( 65)
The distance formula is just the Pythagorean Theorem applied to an x-y graph.
You would get the same final answer if you let (-5, -4) be the second point and (-6, 4) be the first point.