How can I intersect 2 trendlines on the same spreadsheet?

[Guest]

i have 2 trendlines on my scatter graph. they will intersect, but the
trendlines are not long enough. i tried to go into options and change the
forward/backward values. in doing that, however, the spreadsheet goes wacky
in giving me huge figures on the x/y axes. as a result of the change in
coordinates, i am not able to even view the other trendline. HELP!!!!!

 

[Bernard Liengme]

I assume you want to know the position where the two LINEAR trendlines
intersect.
Let the two lines be:
y1=m1x1 + b1 and y2 = m2x2 + b2
At the point of intersection, the two y's are equal as are the two x's
So m1x + b1 = m2x + b2
Hence x = (b2 - b1)/(m1 - m2)
and y = m1*(b2 - b1)/(m1 - m2) + b1
Rather than read the m and b values from the chart, use the SLOPE and
INTERCEPT functions
best wishes