so I have a test in like an hour and I'm confused.
Let's suppose that the hypotenuse is $8$ $\mathrm{cm}$ and our angle is $40^\circ$ and the opposite side is missing.
So $\sin B = \frac{b}{8\textrm{ cm}}$ is $5$ $\mathrm{cm}$ when I multiply.
But when I divide on my calculator I get this $12$ $\mathrm{cm}$.
I was told I had to multiply but $b$ is bigger than the hypotenuse...
Your equation $\sin40^\circ = \frac{b}{8}$ is correct.
So multiply both sides by 8.
$b = \sin(40^\circ)\times 8 = 5.142cm$
EDIT:
Suppose the opposite side was given as 8, and you're told to find the hypoteneuse. That's a different problem. In that case
$\sin(40^\circ) = \frac{8}{h}$
Then multiply both sides by h, and divide both sides by $\sin(40^\circ)$ to get
$h = \frac{8}{\sin(40^\circ)} = 12.45cm$