Billy came up with $\displaystyle {\left[-{2.34},-{2.1}\right]}$ for the mean difference between the number of hours per week students spend doing homework for their English class and their math class. The midpoint of this interval is the sample mean difference. It is calculated as:
$\displaystyle \frac{{-{2.34}-{2.1}}}{{2}}$
Calculate this midpoint.