Singular Values & the SVD · Medium problem

For $A = \begin{pmatrix}3 & 0\\4 & 5\end{pmatrix}$, let $u_1 = \begin{pmatrix}a\\b\end{pmatrix}$ be the first left singular vector (the one for the largest singular value), with $a > 0$. Find $\dfrac{b}{a}$.