What is the difference between random variable and uncertainty?

Question

Please help me to find the difference between random variable and uncertainty? Can use the formula for the area of random variables (expectation, derivation,...)in the field of nonrandom variables (uncertainty with probability)? How can I solve Z=a/(b+s^2) where a,b are uncertainty with probability and s are Laplace variable(without probability) For example a=[2-5]with normal distribution and B=[1-3] normal distribution. Please be careful a,b are Non-random nature but they have probability. Hence, I think we can use formula for the area of random variables to solve problem. Don't you?