$\displaystyle \sum xy -\frac { (\sum x) (\sum y)}{n}$
x= 1, 3, 7, 10
y= 2, 0, 4, 8
What exactly is your question? Do you not understand what the $\displaystyle \sum$ means? It just means "add them up". I assume that n is 4 here.
$\displaystyle \sum_{n=1}^4 x_ny_n- \frac{\left(\sum_{n=1}^4 x_n\right)\left(\sum_{n=1}^4 y_n\right)}{4}$
$\displaystyle = (1)(2)+ (3)(0)+ (7)(4)+ (10)(8)- \frac{(1+ 3+ 7+ 10)(2+ 0+ 4+ 8)}{4}$