I'm sorry, but I'm a bit confused on what you're trying to ask. Could you please clarify?
n = # of days
45 = 2.5n + 2n
45 = 4.5n
n = 10
He can paint 45 structures in 10 days
Step-by-step explanation:
eqn of line : y = mx+ c
m = slope or gradient
c = y-intercept.
Lets take (1,6) = (x1,y1) and (2,3) = (x2,y2)

Substitute x = 1 and y = 6 into equation,

Therefore substituting m and c value, equation of line is

X. 0,2,6,4,8,10
Y. 2,0,2,4,6,8
It’s how far the number is from zero