10x 3y= what? You don't really mean anything

if you mean multiplication then it is
10 x 3y
obviously = 30xy

Not enough information given really.

Are you referring to the linear graph equation y=mx+c?

If so then you would have to reverse this equation to 3y=10x + 0
(I assume it would be +0 since you have not given a value for c)

you'd then have to divide it all by 3, since you have the value of 3y (three 'y's) and not 'y' which is what you want.
10 divided by 3 is actaully 3.333 recurring. And 0 divided by 3 is still 0.
Therefore the answer would be y=3.333(recurring) + 0.

If you mean multiplication, then I agree with the answer given by FBIBosch.

Except that x should never be used for times when you're using variables. So either that isn't it, or it's an improper way to write it.