math
posted by ladybug .
can someone please help me with this question.
Given the funtion f(x)= 3x9
a. find the rate of change between the two stated values for x: 7 to 8
b. find the equation of a secant line containing the given points: (7,f(7)) and (8,f(8))
I think I have (a)answered, just need help with b; please.

f(7) = 219 = 12
f(8) = 249 = 15
so the change in the range = 1512 = 3
change in domain = 87 = 1
for every change of 1 in the x's there will be a change of 3 in the y's
or
rate of change is 3/1 = 3
b) from above, slope = 3
y = 3x + b
sub in (7,12)
12 = 3(7) + b
b = 9
y = 3x  9 ahhh, the same as the original equation !
(since y = 3x9 is a straight line, of course we are going to end up with the same equation for a "secant" between any two points on a straight line)