A staircase can be imagined to be a right triangle. To find the angle of the base corner, you need to identify which sides you know. Since height and length are given, we have the sides that are Opposite to the angle and Adjacent to the angle.
This means that we use the Tangent function.
Since tan(x) = 68.7/95.4
Use the inverse function of ram to solve for x.
tan^1 = (68.7/95.4)
Plug that in to your calculator (make sure it’s in degrees mode)
And you have your answer