Question 469396
Start first with the intercept form for the line.  
The reason for this is, two of the given points are the intercepts of the line.
((p,0) is the x-intercept, while (0,p) is the y-intercept.)

The intercept form is {{{x/a + y/b = 1}}}, where a = b = p.

Hence, after substitution, {{{7/p + 1/p = 1}}}.

==> {{{8/p = 1}}} ==> p = 8, and so the equation is x + y = 8.