a firm invests £10000 in a business which has a net return of £500 per year. An investment of £20000 would yield a net return of £2000 per year. What is the linear relationship between investment and net return? What would be the annual return on an investment of £12000?

If they say it's linear you should be able to find the slope based off investment vs return graphically. Then you could either plot 12000 or calculate its return with the equation of that line from the form y = mx + b