Mass suspended by 3 ropes (midterm practice)

[thesunken]

A 100kg mass is supended by 3 ropes. one rope is attached at a point 1m$\hat{x}$ + 1m$\hat{y}$, one is attached at 1m$\hat{x}$ - 1m$\hat{y}$ and one is attached at -4m$\hat{x}$. The three ropes all connect at -1m$\hat{z}$, at which point the mass is attached. What is the tension T in the rope attached at -4m$\hat{x}$?

I found this problem in my midterm study sheet, but I have no idea how to solve it.
I only know the answer is supposed to be 810N but I don't understand how to get there. Thanks

 

[ehild]

First draw a picture, then construct the free-body diagram.

ehild