This is a question from Degroot and Schervish's Probability and Statistics Textbook: p 103 qn 9

Suppose that a random variable X has a discrete distribution with the following probability function:

f(x) = ( c/(x squared) for x=1,2,...

( 0 otherwise

Find the value of the constant c.

I tried working it out and my answer is c = 1.

But the back of the textbook states the answer as 6 / pi squared.

Please could someone help me understand why?