Answer
$s_{b_1}=10.966$
Work Step by Step
$s_e=269.049$ (item (i))
To find $s_x$:
Using the EXCEL, enter the Size values in the column A.
In A10 enter "=STDEV.S(A1:A9)"
$s_x=8.6747$
$s_{b_1}=\frac{s_e}{\sqrt {∑(x_i-x ̅)^2}}=\frac{s_e}{\sqrt {n-1}s_x}$ (see note on page 684)
$s_{b_1}=\frac{269.049}{\sqrt {9-1}\times8.6747}=10.966$