4a. An iron bar is heated. Its length, L, in millimetres can be modelled by a linear function, L=mT+c, where T is the temperature measured in degrees Celsius (°C).
At 150°C the length of the iron bar is 180 mm.
Write down an equation that shows this information.

4b.
At 210°C the length of the iron bar is 181.5 mm.
Write down an equation that shows this second piece of information.

4c.
At 210°C the length of the iron bar is 181.5 mm.
Hence, find the length of the iron bar at 40°C.

(really need most help on last one)