True active risk calculation

Could somebody help me with the algebra on the below problem. How do you solve for the True active risk. Total active risk = Square root of [(True active risk)^2+(Misfit active risk)^2] 4.8 = square root of [(True active risk)^2+(2.8)^2] Thanks.

4.8^2 - 2.8^2 = true active risk ^2

hahahaha. I was just doing the same problem in 2010 Schweser…