Can anybody tell me why it’s “-275”? Why is it negative ? And what is the formula for finding y? And if y=b how do you find b?

Answer:
275 is the amount SPENT per month for rent. Since you are decreasing that amount from your savings, it is represented with a negative sign.
The formula is: y = mx + b where
If you think about it logically - You have 3500 and spend $275 per month
1st month: 3500 - 275 = 3225
2nd month: 3225 - 275 = 2950
3rd month: 2950 - 275 = 2675
4th month: 2675 - 275 = 2400
5th month: 2400 - 275 = 2125
6th month: 2125 - 275 = 1850
It is easier to write it as an equation where x is the number of months and y is the balance for that month.
y = -275x + 3500
To check if the point (6, 1850) is on the line, input x = 6 and y = 1850
y = -275x + 3500
1850 = -275(6) + 3500
1850 = -1650 + 3500
1850 = 1850
Since it makes a true statement, (6, 1850) is a point on the line.