Question

My question is in the comments. Please help me.

Answer 1

subtract rental cost from the previous and divide by the difference between no of attendees

Answer 2

Difference in Attendees: 15 15
Difference in Rental costs: 135 135

So the relationship is linear (straight line).
Slope = y-difference / x-difference = 135 / 15 = 9

y = mx + b
y = 9x + b
when x = 15, y = 420
420 = 9*15 + b
420 = 135 + b
b = 420 - 135 = 285

y = 9x + 285.

The base price is when x = 0 (zero attendees).
y = 0 + 285 = $285