Think of the right triangle formed by the ladder, the height of the upper end of the ladder and the distance of the bottom of the ladder from the wall.
Then x = 6 ft and the hyp is 10 ft. The trig function cos theta equals "adj side" / hyp, so, in this case,
cos theta = 6/10 = 0.6. Use the arccos x function on your calculator to find the angle x:
x = 0.927 (radians) = 53.1 degrees.