Answer
$a_{max}=4.00$ m/s$^2$

Work Step by Step
For $0 \leq t < 4$ s
$a=\frac{\Delta v}{\Delta t}=\frac{14}{4}=3.5 m/s^2$
For $4 \leq t < 5$ s
$a=\frac{\Delta v}{\Delta t}=0 m/s^2$
For $5 \leq t < 8$ s
$a=\frac{\Delta v}{\Delta t}=\frac{26-14}{8-5}=4 m/s^2$
$a_{max}=4.00 m/s^2$