By the method of least squares, fit y = Bx to the following n=6 points.
x 3 1 5 6 3 4
y 4 2 4 8 6 5
Well, it didn't really matter since your exam is today. If you don't know already, you're probably toast.
I would have preferred y = (36/23) + (41/46)x, with the usual Normal Equations, but you insisted that the intercept is zero, so that limited the choices and the least squares approximation turned out to be y = 1.25x.
Thus: $\displaystyle B \cdot \sum{x^{2}}\;=\;\sum{xy} \implies B\;=\;\frac{120}{96}\;=\;1.25$
Have you met Normal Equations? They ARE important.