We are given height of a slide and aloted ground:
a = 4ft
b = 7ft
To find out the angle we need to use trigonometry. Here we have right-angle triangle. In that case we have lengths of two sides but not hypothenuze. We can use tangens function to find angle:

We need inverse tangens function to find angle:

The angle is less than 35 degrees so it is safe to build it.