<PRE><B>Question 250616</B>
You have two coordinates here, but be careful, we want them in (X,p) form: (42,20) and (52,10). 
step 1: we must find the slope with the formula:
{{{m = (Y2-Y1)/(X2-X1)}}}
{{{m = (20-10)/(42-52)}}}
* m = -1
--
step 2: find b value using Y = mx + b and a point from above. I will choose (42,20)
20 = -1(42) + b
** b = 62
--
step 3: y= mx + b. Put in m and b information.
P = -1X + 62.</PRE>