Slope is always (y2-y1)/(x2-x1) in this case:
(-2-3)/(6--4)
-5/10
-1/2
In mathematical notation, ∑ means summation. So wherever you see the symbol, you have to sum over all elements in the set. Let's solve the question.
a. ∑m means, to sum all elements in the set m: ∑m = 12 + 15 + 20 + 30 = 77
b. ∑
means to sum all elements in the set f as the square of the elements f = 
c. ∑mf means to sum all elements as the product of the elements of m and f = (12×5) + (15×9) + (20
10) + (30×16) = 875
d. ∑
means to sum all elements as the product of the square of m and f = 
This will leave you with
140 = 2(40) + 2x
60 = 2x
30= x
So the length is 30.