You need to find the weighted average. You can also find it on a periodic table :)
6.015*.0742 + 7.016*.9258 = approximately 6.942
On the periodic table, the atomic mass is 6.941, so that is pretty close :)
The average atomic mass is a weighted average, which means that it must take into account the relative abundance of each isotope.
(6.015 amu x 0.0742) + (7.016 amu x 0.9258) = 6.9417 amu