Two Variables · Challenge problem

For a data set, $\sum (x - \bar x)^2 = 10$, $\sum (x - \bar x)(y - \bar y) = 20$ and $\sum (y - \bar y)^2 = 50$. A student draws a line through the point $(\bar x, \bar y)$ with slope $3$ instead of the least-squares slope. By how much is the student's sum of squared residuals larger than that of the least-squares line?