struggling with where to begin with this one, covariance and variance always has confused me

4. The length of life Y for fuses of a certain type is modeled by an exponential distribution, with pdf g(y) = e -y , y > o , elsewhere. Recall that E(Y) = 1, and V(Y) = 1. Suppose that there are two such fuses with independent lengths of life Y1 and Y2. (a) (4 points) Find the covariance of Y1 and Y2. (b) (5 points) Suppose that one fuse is in a primary system, and the other is in a backup system that comes into use only if the primary system fails. The total effective length of life of the two fuses is then Y1 + Y2. Find the expected value and variance of Y1 + Y2. (c) (10 points) Find P(Y1 + Y2 o , elsewhere. Recall that E(Y) = 1, and V(Y) = 1. Suppose that there are two such fuses with independent lengths of life Y1 and Y2. (a) (4 points) Find the covariance of Y1 and Y2. (b) (5 points) Suppose that one fuse is in a primary system, and the other is in a backup system that comes into use only if the primary system fails. The total effective length of life of the two fuses is then Y1 + Y2. Find the expected value and variance of Y1 + Y2. (c) (10 points) Find P(Y1 + Y2