Set up two simple equations then solve like you would any two-equation system.
You only have two isotopes, which means that the sum of their respective abundances will amount to 100%, or to 1 if you use fractional numbers.
Let's say the abundance of
Now solve this like you would any two-equation system.
This will get you
Plug this into the first equation to get
Expressed in percentages and rounded to four sig figs, these values will be
Finally, check the result to see if it makes sense. Notice that the average atomic mass of the two isotopes is 6.941, which is closer to 7.0160, the atomic mass of the heavier isotope,
This means that the heavier isotope is much more abundant than the other, which is confirmed by the abundaces we got - 92.5% for the heavier one and only 7.49% for the lighter isotope.