Two Population Means with Known Standard Deviations · Exercise 1

The mean speeds of fastball pitches from two different baseball pitchers are to be compared. A sample of 14 fastball pitches is measured from each pitcher. The populations have normal distributions. shows the result. Scouters believe that Rodriguez pitches a speedier fastball.
$$\begin{array}{ccc}\text{Pitcher}& \text{Sample Mean Speed of Pitches (mph)}& \text{Population Standard Deviation}\\ \text{Wesley}& \text{86}& \text{3}\\ \text{Rodriguez}& \text{91}& \text{7}\end{array}$$

What is the random variable?

OpenStax, Introductory Statistics 2e · §10.2 Two Population Means with Known Standard Deviations · Exercise 1