Drug test probability

Mumble.com uses tests with a 5% false positive and a 10% false negative. A mandatory second testing is being done.
a. if one employee is selected at random, what is the probability that the selected employee uses drugs and tests positive twice?
b. what is the probability that the employee tests positive twice?
c. if we know that the randomly chosen employee has tested positive twice, what is the probability that he uses drugs?

Have you tried any work? Start by drawing a tree diagram with the probabilities.

I have the probabilities of the first test but do I just double the original probability?

I assume that you also have the probabilities of an employee using or not using drugs as well and just posted insufficient information. But yes, the 2 tests are independent.