If this function is linear, write it in intercept slope form.
So, this question is a tad bit confusing. Is there more to the problem?
If not, and your equation is:
Then there really is nothing you have to do to write it in slope-intercept form (y=mx+b).
y=mx+b, where m=slope, b=y-intercept (where the line crosses the y-axis)
For your equation, you can think of c(r) being 'y' and r is 'x'. If 'p' is just a number, then I'd say,
m=3p and b=0
So, essential it already is in slope-intercept form, or you can re-write it with y and x
But I am wondering if there is some information missing from this problem....