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