We can simplify both the numeric and literal parts.
It could be useful to rewrite the fraction as follows, to make clear what we're simplifying with what:
[tex]\dfrac{28st}{-4t} = -\dfrac{7\cdot 4}{4}\cdot s \cdot \dfrac{t}{t}= -7\dfrac{4}{4}\cdot s \cdot \dfrac{t}{t}[/tex]
The fractions 4/4 and t/t simplify to 1, since the numerator and denominator are the same. So, we're left with
[tex]\dfrac{28st}{-4t} =-7s[/tex]
