let

x = the cost of the tennis racket

y = the cost of the book

tennis racket costs 3 times as much as a book

x = 3*y

the total cost of the book and the tennis racket is $36

x + y = 36

by solving the system of equations

x = 3*y

x + y = 36

we find

x = $27

y = $9

The cost of the tennis racket is $27.