Matched or Paired Samples · Exercise 12

A doctor wants to know if a blood pressure medication is effective. Six subjects have their blood pressures recorded. After twelve weeks on the medication, the same six subjects have their blood pressure recorded again. For this test, only systolic pressure is of concern. Test at the 1% significance level.
$$\begin{array}{ccccccc}\text{Patient}& \text{A}& \text{B}& \text{C}& \text{D}& \text{E}& \text{F}\\ \text{Before}& \text{161}& \text{162}& \text{165}& \text{162}& \text{166}& \text{171}\\ \text{After}& \text{158}& \text{159}& \text{166}& \text{160}& \text{167}& \text{169}\end{array}$$

What is the test statistic?

Its source prints no answer for this problem, so there's nothing to check against.

OpenStax, Introductory Statistics 2e · §10.4 Matched or Paired Samples · Exercise 12