$Did you end up with $\arcsin{(3x+1)}=\arccos{x} \to \arcsin{(3x+1)}=\arcsin{(\sqrt{1-x^2})} \to 3x+1=\sqrt{1-x^2}?
If you squared at this point, you end up with incorrect solutions. Assuming you did, you will get x=0,-\frac{3}{5}. If you sub x=-\frac{3}{5} into 3x+1=\sqrt{1-x^2}, you'll see...