Assessing the fit of a line

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Problem

In order to earn a little extra money, Anna set up a lemonade stand on her block over the summer. Suspecting that there might be a relationship between the temperature and the amount of lemonade she sells, Anna recorded the day’s high temperature and the number of cups of lemonade she sold for 10 days. After plotting her results, Anna noticed that the relationship between her two variables was fairly linear, so she used her data to calculate the following least squares regression equation for predicting lemonade sales from the daily high temperature:
$\hat{y} = -33.704+0.6012x$
What is the residual if a day had a high temperature of 94 degrees and Anna sold 21 cups of lemonade?