1. **State the problem:** We need to find the formula for cost based on the mass of coal, given by the linear equation: $$\text{cost} = m \times \text{mass of coal} + c$$ where $m$ is the gradient (rate of change of cost per tonne) and $c$ is the y-intercept (fixed cost). 2. **Identify points from the graph:** The line passes through points $(0, 80)$ and $(2, 400)$. 3. **Calculate the gradient $m$:** $$m = \frac{\text{change in cost}}{\text{change in mass}} = \frac{400 - 80}{2 - 0} = \frac{320}{2} = 160$$ 4. **Find the y-intercept $c$:** From the point $(0, 80)$, the cost when mass is zero is $80$, so: $$c = 80$$ 5. **Write the formula:** $$\text{cost} = 160 \times \text{mass of coal} + 80$$ 6. **Interpretation:** This means there is a fixed cost of 80, and for every tonne of coal, the cost increases by 160. **Final answer:** $$\boxed{\text{cost} = 160 \times \text{mass of coal} + 80}$$