1/u+i/v=2/s as u being sudject of fomulae
Do you want an explicit equation for u? Is that really i/s or did you mean 1/s on the left side?
If you meant 1/s, then
1/u = 2/s - 1/v = (2v -s)/vs
Now take the reciprocal of both sides. (Turn the fractions upside down)