Engineering Probability and Statistics!?

The integral of a probability density function must be equal to one, so taking the integral of k*x^3 you get (k/4)*x^4, taking that between bounds of 0 and 1 you k/4 you then set that equal to one and get that k=4, to get the probability's you take the integral of 4*x^3 between the bounds of 3/4 and 1/4, which I calculate to be .3125, and for greater then 2/3 you take the integral from 2/3 to infinity, which can simply be from 2/3 to 1 since anything greater then one is zero, I calculate this to be.80246914