216° was given as the reflex angle of the arc, and 15cm as the radius. I used these things to find the length of the arc by obviously doing:
pi x d (30cm) x 216/360
I believe that the arc length would give me the actual circumference of the base, because when the net is folded, that would be the bottom edge. So if you do the calculation for the arc length above, you find the circumference of the base must be 56.54866776.
So I did
56.54866776= πd
18=d (divded by pi)
so base r= 9
I then just drew a right angled triangle with the side slope (15cm), the radius of the base (9cm) and the height that was asked for. And used pythagorases theorem like this
15²-9²=144
√144=h
h=12.