Help! Help! Help! Gcse Maths Coursework - Number Stairs

Hi I am currently studying GCSE Maths intermediate tier - AGAIN and have reached a ded end with my number stairs coursework. I have found the nth term for 6 squared stair shapes on a 10x10, 9x9, 8x8, and 7x7 grid, using the top left number. I need to find the nth term formula that generates the top left number on any sized grid for a 6 squared stair shape.
The nth term formula for the 10x10 grid is - 6n+84
" " 9x9 grid is - 6n+76
" " 8x8 grid is - 6n+68
" " 7x7 grid is - 6n+60
If any1 has done this piece of coursework can they please say what they did, cos its really buggin me. Hope i have explained it right - thankyou