It is given that the radius increases at the rate of 7 cm per second, that is 7 cm in 1s.
It follows that after 4s since backing away, the increase in radius is:
[tex]7\times4=28\text{ cm}[/tex]Add the increase to the initial length of the radius:
[tex]28+5=33\text{ cm}[/tex]Hence, the radius 4s after you start backing away is 33 cm.
After 7s, the increase in radius will be:
[tex]7\times7=49\text{ cm}[/tex]Add this increase to the initial length of the radius:
[tex]49+5=54\text{ cm}[/tex]Hence, the radius 7s after you start backing away is 54 cm.
