Hello,

To calculate the rate of INCREASE in area, you need to differentiate the equation with respect to t. so d(A)/dt will come out to be 6t+1/5+0. (Please note that the r in your question will be t instead. In case that really is r, then revert back in the comments below. There will be other procedure for the same).

After differentiation, you need to put the value of t= 5 seconds. So you will obtain an answer of 6*5+1/5= 30 1/5 is your answer.

Hope this helps.