Alright, so first set up the equation a^2+b^2=c^2
In this equation AR equals a, so put 7 in for a. GA equals c, so put 10 in for c.
You get 7^2+b^2=10^2
From that you get 49+b^2=100
Minus 49 from both sides to get b^2=51
We know that the square root of 51 will be an irational number, so GR is the square root of 51.
You can solve it from there.