The "i" in this problem actually doesn't represent imaginary numbers. This is a summation problem which is asking for the sum of (2i+1) for when i = 1 to i = 7.
So (2(1) + 1) + (2(2) + 1) + (2(3) + 1) + (2(4) + 1)... etc...
The summation rules we need to know for this problem are for constants and for a single variable.
When we have a constant, the summation rule is n*c where n is the number of times.
When we have a single variable, the summation rule is n(n+1)/2.
So applying the summation rules where n = 7:
2(7)(7+1)/2 + 1(7)
(7)(8) + 7 = 63
The answer to the summation is 63.