Multply 400 hudred to the four sides on the lieght
The series is an arithmetic series with first term -10 and common difference (-2-(-10))=8. The general term a_n can be written as
a_n = -10 + 8(n-1)
or
a_n = -18+8n
The last term corresponds to
110 = -18+8n
128 = 8n
16 = n
So, the summation can be written as
Y = mx + b
m = (y2 - y1)/(x2 - x1)
y = (26,600 - 18100)/(20 - 10)x + b
y = 850x + b
18100 = 850(10) + b
b = 9600
R(x) = 850x + 9600