Question

How did we get t^4/4 to t^3?

Answer 1

Looks like differentiation. without going into the reasons why, when you differentiate a term in the form
$a \times {x}^{b}$
it turns into
$a \times \frac{ {x}^{b - 1} }{b}$
as a hint you could rewrite the original term as
$\frac{1}{4} \times {t}^{4}$