The ammount you earn can be describe the following way.
Let x be the number of hour you babysit. we know that for the first three hours you charge $50 and then you charge an extra $10 dollars per hour.
Then the amount you charge in total (that we are going to call y) is described by
[tex]\begin{gathered} y=10(x-3)+50 \\ for \\ x>3 \end{gathered}[/tex]Notice that we restrict the value of x to match the description of the problem (we only charge the extra $10 if we babysit more than three hours).
With this equation we can know how much to charge for five hours (since, in this case x=5>3).
[tex]\begin{gathered} y=10(5-3)+50 \\ =10(2)+50 \\ =20+50 \\ =70 \end{gathered}[/tex]Then, for five hours you would earn $70.