What is the equation of the line containing points 7,3 14,6 and 21,9

Let's present a solution for the generic case.
The equation will be of the form y=ax+b

If (7,3) is on it, then 3=7a+b
If (14,6) is on it, then 6=14a+b

Now we have a system we can solve.

3=7a+b => b = 3-7a
6=14a + 3-7a => 3 = 7a => a=3/7

b  = 3-7*3/7 = 0

So y = (3/7)x is the equation.

Distributive property