Tolong kasih caranya no 12&13

[tex]f(x) = 12 - 3x \\ f(a) = 18 \\ penyelesaian \\ f(a) = 12 - 3x \\ 18 = 12 - 3a \\ 6 = 3a \\ a = 6 \div 3 \\ a = 2[/tex]