Need an explanation how to calculate Return Of Investment for Test Automation

simple task

I have this data and i need to calculate RoI for it. But i'm new to this and i don't know how exactly should i do this. Should i calculate it for every day or just total cost of (manual test - automated test) / automated test (something wrong with the answer if i do it this way? Or do i need to calculate RoI for equal tasks like development of tests, executing and maintaining?