Like this:

C being the distance from the stand the arrow must travel to a target
a= ground distance
b= height distance

c^2 = a^2 + b^2 ... formula to get the distance in a hypotenuse

SO.....

c^2 = 30^2 + 15^2
c^2= 900+ 225
1125= c^2
C=√1125

So your toal distance from 30 yards @ 15 ft high to target would be....

33.54 yrds