y = x + 4

Compare that to

y = mx + b

b = 4 and m = 

The slope m always has three things:

1. A sign, either + or - 
2. A numerator
3. A denominator (If the slope is a whole number is has denominator 1).

Instructions:

Find b on the y-axis. Put a point there:



From that point draw a vertical line the same number of units
long the numberator is.  If the slope is positive, draw it upward and if
it is negative, draw it downward, 

In this case the slope is positive and the numerator is 1 so we draw
a vertical line upward from the point we drew at 4 on the y-axis.

That is indicated by the green vertical line below:



Now from the end of that green line, draw a horizontal line to the right
(always to the right) the same number of units long the denominator is:

In this case the denominator is 3 so we draw a horizontal line 3 units to
the right.  That is indicated by the green horizontal line below:




Now take a ruler and draw a line through the point you started at on the
y-axis and also through the end of the horizontal green line:



That's it.

Edwin