Answer
$\angle A=\dfrac{\pi}{8}$
$a\approx392.65$ $;$ $b\approx162.64$
Work Step by Step
The triangle is shown in the attached image below.
Two angles are known. Let $\angle A$ be the unknown angle, $\angle B$ be the angle marked as $\dfrac{3\pi}{8}$ and $\angle C$ be the right angle. Since $\angle A+\angle B+\angle C=\pi$, substitute the known angles into the formula and solve for $\angle A$:
$\angle A=\pi-\dfrac{\pi}{2}-\dfrac{3\pi}{8}=\dfrac{\pi}{8}$
Use the sine trigonometric ratio of the angle marked as $\dfrac{3\pi}{8}$ to obtain the unknown cathetus marked as $a$:
$\sin\dfrac{3\pi}{8}=\dfrac{a}{425}$
$a=425\sin\dfrac{3\pi}{8}\approx392.65$
Use the Pythagorean Theorem to obtain the cathetus marked as $b$:
$b=\sqrt{425^{2}-392.65^{2}}\approx162.64$
