Brilliant! $$A_y$$ is negative. So, wherever you find $$A_y$$, replace it with its value. Also, if there are other operations on $$A_y$$, like squaring it, etc, use brackets around $$A_y$$.
Just to recap, if $$\vec{A} = A_x \hat{i} + A_y \hat{j}$$, then, the magnitude is just $$\sqrt{A_x^2 + A_y^2}$$. The angle is calculated by taking the inverse tan of $$\frac{A_y}{A_x}$$