Sonbra,
You have not provided many clues, but I suspect that the equation you require is the lift equation.
Lift equals Cl 1/2Rho Vsquared S.
If we keep Rho and S constant then to maintain constant lift for any changes in Cl we need an equal and opposite change in V Squared.
So for example if we half Cl from say 0.5 to 0.25, then we must double V squared. To achieve this we would multiply V by the square root of 2.