Did you ever do this if so do you have an email I can message you on. I've been stuck on this.
Answer:

Step-by-step explanation:
Let sides of the triangle be 

Semi-perimeter (s) = 

Area of one tile 

So,
Area of 20 tiles 
Cost of polishing the floor at the rate of 2 rupees per sq.cm = 
notice the first term is -10, and the common difference is 3, so we add 3 to get the next term.

check the picture below.
(-1, 5) because if you plug in -1 to both equations, you should get 5 from both equations. They also cross at that point, making that the solution.