Simply put, each isotope will contribute to the average mass of the element proportionally to its abundance.
#color(blue)("avg. atomic mass" = sum_i ("isotope"_i xx "abundance"_i))#
In your case, you know that the average atomic mass of chlorine is equal to
#-> " 34.969 u"#and #75.77%#abundance
#-> " ? u"#and #24.23%#abundance
Before doing any calculation, take a second to look at the values you have for the abundances of the two isotopes.
Notice that chjlorine-35 is considerably more abundant than chlorine-37, which means that the average atomic mass of the element will be closer in value to the atomic mass of chlorine-35.
Moreover, the atomic mass of chlorine-35 is smaller than the average atomic mass of chlorine, which means that you expect the atomic mass of chlorine-37 to be bigger than the average atomic mass of chlorine.
When calculating the average atomic mass of an element, it's useful to use decimal abundances, which are simply percent abundances divided by
#"35.446 u" = "34.969 u" xx 0.7577 + x * 0.2423#
Now all you have to do is solve this equation for
#x * 0.2423 = "35.446 u" - "34.969 u" xx 0.7577#
#x = color(green)("36.938 u")#
I'll leave the answer rounded to five sig figs, despite the fact that you only have four sig figs for the abundances of the two isotopes.