Most graphing calculators will do weighted averages pretty easily. It is mostly a matter of data entry.
mx = -2
my = 10
(x, y) = (mx, my)/10 = (-0.2, 1)
Which ever one of those that doesnt have a decimmal
You subtract the intial from the final. your answer is0.94grams
Y=3.4x
2*3.4=6.8
5*3.4=17
8*3.4=27.2
11*3.4=37.4