Start at the given point (2,1)



The gradient is 3.  Consider it as the fraction 3/1.

Since the gradient is positive, we draw a vertical line UPWARD
the amount of the NUMERATOR of the fraction of the gradient,
which in this case is 3, since the gradient is 3/1 with
numerator 3. 

(If it were negative we would have drawn it DOWNWARD)



Next we draw a horizontal line RIGHTWARD 
the amount of the DENOMINATOR of the fraction of the gradient,
which in this case is 1, since the fraction of the gradient,
which in this case is 1, since the gradient is 3/1 with
denominator 1. 

(If it were negative we would STILL have drawn it RIGHTWARD, we ALWAYS
draw the horizontal line RIGHWARD)



Then we draw the desired line through the given point and the point
at the end of the horizontal line:



Edwin