a. Since [tex]a\equiv11\pmod{19}[/tex], we have
[tex]c\equiv13a\equiv-6a\equiv-66\equiv10\pmod{19}[/tex]
so [tex]c=10[/tex]
b. [tex]b\equiv3\pmod{19}[/tex], so [tex]8b\equiv24\equiv5\pmod{19}[/tex], so [tex]c=5[/tex].
c. [tex]a-b\equiv11-3\equiv8\pmod{19}[/tex], so [tex]c=8[/tex].
d. [tex]7a+3b\equiv86\equiv10\pmod{19}[/tex]
e. [tex]2a^2+3b^2\equiv269\equiv3\pmod{19}[/tex]
f. [tex]a^3+4b^3\equiv1439\equiv14\pmod{19}[/tex]