The indicated sum is 350. The answer is 350 because the E or sigma is making you do (6j+2) ten times. Every time you do 6j+2 you start with j as 1 and add a 1 to j for every time you add. For example, [6(1)+2]+[6(2)+2]+[6(3)+2]...[6(10)+2].
Hope this helps.