P(A∩B)=P(A)P(B) if and only if A and B are independent. This is why you get the result that you do, since P(A|B) is just P(A) since A won't depend on B.
For the first part, you can take constants outside the integral, so you can divide both sides by 3. Swapping the limits results in multiplying the integral by -1, giving the required result. For the second part, you can split the integrals up into an integral you know how to integrate, and the integral given in the question.
For the first part, you can take constants outside the integral, so you can divide both sides by 3. Swapping the limits results in multiplying the integral by -1, giving the required result. For the second part, you can split the integrals up into an integral you know how to integrate, and the integral given in the question.