How do I rearrange DeBroglie's Wavelength equation to solve for velocity?

Multiply both sides by velocity.
Then divide both sides by wavelength.

The asker asked how to SOLVE for velocity.... how are they supposed to multiply each side by velocity if the velocity is unknown.,,,?>

Solve for velocity means that we modify the equation so that from the one side we will only have velocity. For exaple, the equation:
velocity=Plank's constant/(mass*wavelength)
we say that we solved the equation for velocity, because from the one side we have only velocity. (we usualy do this to FIND the velocity value, if we know the values of the other variables.)

It IS possible to multiply/ divide (assuming that it's not 0)/ add/ abstract an unknown number from both sides of a equation.
For example: a/b=c <=> (a/b)*b=c*b <=> a=c*b
b is unknown but we still managed to multiply both sides by it.....