How do you write an equation for (0,4) (2,8)

You need to first solve for the slope and then the y intercept to put the equation in slope intercept form (y=mx+b).  The slope is "m" and "b" is the y intercept.

To find the slope use the equation-
y2-y1/x2-x1
(x1, y1) is one point and (x2, y2) is another point
8-4/2-0
4/2
2=m/the slope

Now solve for b/the y intercept by plugging in a point's coordinates (x,y) and the slope-
8= 2(2)+b
8=4+b
-4  -4
4=b
Now plug in the b in slope intercept form-
y=2x+4

Hope this helped! :)