A12-ft-by-15-ft swimming pool has a 3-ft-wide no-slip surface around it. what is the outer perimeter of the no-slip surface?

see diagram

we see we doulbe the 3 to add
3*2=6

6+12=18
6+15=21
thelength and width of the surfface is 18 by 21

P=2(L+W)
P=2(21+18)
P=2(39)
P=78

the perimiter is 78ft