Remeber that given a proposition of the following form:
[tex]p\Rightarrow q[/tex]then, the contrapositive of the implication is:
[tex]\neg q\Rightarrow\neg p[/tex]in this case, we have the following:
[tex]p\Rightarrow q\equiv\text{ If Mike does his Geometry homework, then he will get a good grade}[/tex]then, the contrapositive is:
[tex]\begin{gathered} \neg q\Rightarrow\neg p\equiv \\ \text{If Mike does not get a good grade, then he did not do his Geometry homework} \end{gathered}[/tex]