Find the sum of the summation of 2 i minus 12, from i equals 7 to 16?

Break up the summand as follows: 2 i minus 12 becomes

2 (summation of i from 7 to 16) - (summation of 12) (16-7)

This comes out to 2[ (summation of i from 1 to 16) - (summation of i from 1 to 7) ] - 12 [summation of 1 from 7 to 16]