A lamp has two bulbs, each of a type with an average lifetime of 4 hours. The probability density function for the lifetime of a bulb is http://mathcs.chapman.edu/webwork2_f...e2835d8ac1.png.

What is the probability that both of the bulbs will fail within 4 hours?

i took the douple integral of this function with bounds from 0 to 4 and got the answer 1 which isn't right where did i go wrong?