Have the following continuous random variable....

Hi I know this is a relatively easy question but im a bit stuck, previous one i have calculated i have found the cumulative function and plug in the upper limit and equal it to 1 and from the find the constant. Now with this having no upper limit im not to sure. The rest of the question i know how to calculate, but you need to find the constant in order to work it out, the question is as follows:

f(x)=c/(x^3) for x greater or equal to 1

0 otherwise.

1) find the value of the constant c.

Many thanks in advance.