Lithium has two isotopes.
Li-6 and Li-7.
Li-6 mass = 6.015121amu
Li-7 mass = 7.016003amu
Lithium Mass = 6.94049amu

What is percent abundance of Li-6?

I know that there is an equation:
6.9409 = (% Abundance Li-6 * Weight of Li-6) + (% Abundance Li-7 * Weight of Li-7)

I don't know how to use it.