draw a right-triangle with acute angle 2x and opposite side 24 and adjacent side 7. use Pythagoras' theorem to find the hypotenuse.

then you will know that cos(2x) = (adjacent)/(hypotenuse)

then use the fact that to find sin(x) and cos(x)

or you could solve.

tan(2x) = -24/7

=> sin(2x)/cos(2x) = -24/7

=> 7 sin(2x) = -24 cos(2x)

and use the double angle formulas to change everything to sine and cosine and continue