The cosine of the angle in a right triangle is (adjacent side) divided by (hypotenuse).
As luck would have it, the picture doesn't give us the length of the adjacent side, so we have to calculate THAT before we can calculate the cosine.
You can easily find the length of the adjacent side ... call Pythagoras in. The adjacent side is the square root of (10-squared minus 2-squared).
When you have that, the cosine of the angle is just (that) divided by 10.
