If a graph is a function, it will always have y=... or x=... (or anoher letter equals an equation) for example y= 3x-12 is a function
The lines are parallel.
I don't understand your question but y=3x is the function of a graph, to graph the function you would plug points into the function such as x=0, x=1, x=-1 and you would find the y values at each point so that you can graph it. In this case the graph is a parabola which has a u shape.
They are the same equation just written differently so they are both easy to graph.
If you mean: y = 15+3x then it is the function of a straight line equation.