The increase of the radius is a linear increase since we have the constant rate of 0.07 inches per second
The equation for a linear growth/decay is given by the form
where 
is the rate of increase and 
is the value of 
when 
We have
when 
So the equation is
The length of the radius when
seconds is
inches
The equation for a linear growth/decay is given by the form
We have
So the equation is
The length of the radius when