Three forces $F_1^{ },F_2^{ }$ and $F_3^{ }$ can  keep the body in equilibrium. If $F_1^{ }$ = 3N acts along positive x-direction and $F_2^{ }$ = 4N acts along positive Y-direction , then angle made by $F_3^{ }$ with negative Y-axis is ${\tan }^{-1}\left(\frac{x}{y}\right)$ then x+y is_________