You didn't give an original equation, just the two points so I'm just going to go ahead and assume that b = 0, but in reality, it doesn't really matter what it equals I think.
Slope intercept form:
First, we find the slope, so we do y1-y2/x1-x2 which is 3. So then we do y=mx+b so 4=3(2)+b. Then we do 4=6+b. So, b = -2.
So the equation is y=3x-2
Find the slope which we already know. Plug into y-y1=m(x-x1).
y-4=3(x-2)
y-4=3x+6