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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 \(\frac{A_y}{A_x}\)