This is going to involve trig functions.
you have the angle, and if you draw a triangle and fill in the numbers, you'll see that the length of the slide is the hypoteneuse of this triangle.
You are looking for the height of the triangle, which is the side opposite of the angle.
The function cosine will help you here. Sine = opposite over hypoteneuse.
sin29 = x/5.4
x = 2.618
The top of the slide is 2.618 meters above the ground.