Equation for a Right Angled Triangle
$C = \sqrt{A^2 + B^2}$
If you know $C$
$C = 49$
And you know the ratio of width to height
$16:9$
Is it possible to work out $A$ and $B$ or will there be too many answers. I can't imagine there will be because the ratio makes it pretty strict.
WLOG, let $A > B$
Then the ratio implies that $9A = 16B\implies A=\frac{16}{9}B$
Substituting this in:
$$49=\sqrt{(\frac{16}{9}B)^2+B^2} = B\sqrt{\frac{337}{81}} $$
$$B = \frac{49\cdot 9}{\sqrt{337}} = \frac{441}{\sqrt{337}}$$
Then $A = \frac{784}{\sqrt{337}}$