see the picture attached to better understand the problem
we know that
in the right triangle ABC
cos 70°=adjacent side angle 70°/hypotenuse
adjacent side angle 70°=AB
hypotenuse=AC-----> 25 mm
so
cos 70°=AB/25-----> AB=25*cos 70°------> AB=8.55 mm
the answer is
the side adjacent a 70 degree angle is 8.55 mm long
