TeX source:

area of C = (\text { area of } \mathrm{A})-(\text { area of } \mathrm{B})=\int_{\mathrm{a}}^{\mathrm{b}} \mathrm{f}(x) \mathrm{dx}-\int_{\mathrm{a}}^{\mathrm{b}} \mathrm{g}(x) \mathrm{dx}=\int_{\mathrm{a}}^{\mathrm{b}} \mathrm{f}(x)-\mathrm{g}(x) \mathrm{dx}