You did well by solving the quadratic, and you seem to have spotted that cos(x) can't be 2 because the cosine function can't give a result greater than 1 or less than -1. x=arccos(1/4) or x=cos^-1(1/4) is how the answer is found. This may be a function on your calculator. If you use tables you would look in the body of a cosine table to find what angle gives you a cosine of 0.25. The answer is 75.52 degrees approximately.