lean_workbook_plus_21675

I think the simplest way is to use mods. First of all, note that $3^3=27\equiv-1\pmod{28}$ . Hence, $3^{114}=(3^3)^{38}\equiv(-1)^{38}\equiv1\pmod{28}$ . Thus, $3^{115}=3(3^{114})\equiv3\pmod{28}$ .