for each equation find x when y=15

[fallinbehind]

for each equation find x when y=15

*is really pi, there is no pi button on my keyboard

Here is a sample problem: y=4*x^2

 

[Gene]

Just type it as 4*pi*x^2
15 = 4*pi*x^2
x^2=15/(4*pi)
x=sqrt(15/(4*pi)) =
(1/2)*sqrt(15/pi)