The quoted is the weighted average of the individual isotopic masses of $""^7Li$ and $""^6Li$.
$6.941$ $"amu"$ $=$ $7.015*"amu"xx92.58%+("mass of "^6Li" isotope")xx7.42%$
And thus, $"6.495 amu"+("mass of "^6Li" isotope")xx7.42%=6.941*"amu"$
And thus,
$("mass of "^6Li" isotope")xx7.42%=(6.941-6.495)*"amu"$
$("mass of "^6Li" isotope")xx7.42%=0.446*"amu"$
$"mass of "^6Li" isotope"=(0.446*"amu")/(0.0742)=6.011*"amu"$.
How did I know that $""^6Li$ had an abundance of $7.42%$?