In MS Excel, I need to know how to calculate the year in which two
lines intercept. For example: at year 2.45, R Cumulative intercepts P
Cumulative. See the table below for details. I have built a line chart
using the data below in MS Excel, and the value appears to be approx
2.45, but I don't know which formula to use to be sure.

Year	1	2	3	4	5

Rent	 $47,916 	 $47,916 	 $47,916 	 $47,916 	 $47,916 
R Cumulative	 $47,916 	 $95,832 	 $143,748 	 $191,664 	 $239,580 
					
Purchase	 $95,980 	 $14,397 	 $14,397 	 $14,397 	 $14,397 
P Cumulative	 $95,980 	 $110,377 	 $124,774 	 $139,171 	 $153,568 

Intercept	 $81,583 				
Year Intercept	?

The formula of a straight line is

y = m x + c

So you want to know when y1 = y2

y1 = 47916 x + 0
y2 = 14397 x + 81583

Simplifying

(47916 - 14397)x = 81583

33519 x = 81583

x = 2.43

In general terms

x =  c2 - c1      difference in initial payments
     -------
    (m1 - m2)     difference in regular payments

Note the different order of terms