\[f(x) = 5x + 1;\ \ \ 3 \leq x \leq 6.\]
\[f(3) = 5 \cdot 3 + 1 = 16;\]
\[f(6) = 5 \cdot 6 + 1 = 31.\]
\[E(f) = \lbrack 16;31\rbrack.\]