(a,b) must lie on the line through (3,0) and (4,8), which is
the line below.  The red points labeled (a,b) are several of 
the infinitely many possible points that (a,b) could be:



So we find the equation of that line using the
slope formula, and then the point-slope formula:

The slope formula is

m = 
where (x1,y1) = (3,0)
and where (x2,y2) = (4,8)

m =  =  = 8

The point-slope formula is

y - y1 = m(x - x1)
where (x1,y1) = (3,0)

y - 0 = 8(x - 3)
    y = 8x - 24

We can substitute x=a and y=b and get an equation

    b = 8a - 24

which is true for ANY point (a,b) on the above line,
[including the two given points!]

Edwin