l = V / (wh)
Given just the variables of this equation, to solve for length (l) simply means to isolate it or to get l on one side of the equation by itself.
We can do this by solving for it as any typical equation. On one side we have V, on the other we have lwh or l*w*h.
To move wh to the other side we must divide both sides of the equation by wh:
V/(wh)=lwh/(wh)
Since wh/(wh) on the right side will just cancel and become 1, we are left with just l on the right side, thus;
V/(wh) = l