method 1 ...

use the Pythagorean identity

sub in the value of sin(m) and solve for cos(m). remember that cosine < 0 in quad II

method 2 ...

sketch a reference right triangle in quad II ... label the opposite side 5 and the hypotenuse 7. use Pythagoras to calculate the length of the adjacent side.

cos(m) = (adjacent side)/(hypotenuse) ... remember cos(m) < 0