Since you have the side opposite of x and the hypotenuse, and [tex] \sin(x) = \frac{opposite}{hypotenuse} [/tex] you have
[tex] \sin(x) = \frac{66}{79} [/tex] Now solve for x. [tex]x = \arcsin( \frac{66}{79} ) [/tex] Another way to write arcsin() is [tex] \sin^{ - 1} () [/tex] In either case, plug that into your calculator and you'll find that [tex]x = 56.7 ^{o} [/tex] Make sure your calculator is in degree mode and not radian mode.
