The graph of a company's profit $P(t)$ in dollars, at month $t$ is shown.

Complete parts a and $b$ below.

a$.$ Estimate, to the nearest whole numbers labeled on the horizontal axis, any values of $t$ where the second derivative is zero.

$t=($Type a whole number. Use a comma to separate answers as needed.$)$

b$.$ List any values of $t$ where the second derivative is the same as the first derivative.

$t=($Type a whole number. Use a comma to separate answers as needed.$)$