"x is an exponential random variable with mean equal to 100. what is the probability that x equals 90? "

Since mean is 100 so Λ=1/100,

But since exponential distribution is continuous distribution so probability at a particular points  

Is zero. That probability  

That X is equal to 90 degree is 0.

let m = monomial

let t = trinomial

so, m times t = t.

monomial times a trinomial = another trinomial.