Question

how to graph? 3x-6y=12?

Answer 1

take any value of x, say 1. substitute the value for x in the equation to get y's value. no choose another x, say 0, get y. do that another time.
note down the corresponding coordinates of each set. plot them down on the graph with proper scaling. and then, draw a line connecting all the points.
(although you need just two points to draw a straight line, the third one is for verification and to minimize mechanical errors due to thickness of pencil tip etc).