A general linear equation that relates T and D is T = aD + b for some constants a and b. Substitute the two given pairs of values of T and D into that equation, and you'll get two equations in a and b.
My old Holden’s speedometer was not calibrated correctly it displayed70 km/h when its true speed was 55 km/h, it also displayed 40 km/h whenits true speed was 32 km/h.
(a) Using the variables T for true speed and D for displayed speed, determine the linear equation that could relate the displayed and true speeds.
(b) When the speedometer displays 100 km/h how fast am I actually going?
(c) When I am going 100 km/h what speed does the speedometer display?
The equations are correct. Subtract the second equation from the first one; this eliminates b and leaves one equation in a only. Find a and then substitute it into any of the original equation to get an equation in b only.
For more on solving systems of linear equations, see, e.g., here.