Use the slope-intercept form y = mx + b. You are given b=2.5 and are told that in one case, y = -4.25 and x = 4.5.
Then y = mx + b becomes -4.25 = m(4.5) + 2.5
Multiply all 3 terms by 100: -425 = m(450) + 250. Subtract 250 from both sides. The result is -175 = m(450). Solving for m,
m = -175 / 450 = -7/18
Then the equation is y = (-7/18)x + 2.5
