what happen to the graph of the line y=3x-2 when equation is changed to y=3x+6

The y-intercept changes from -2 to 6. Graph them both and see.

you've given two equations in slope-intercept form.
slope-intercept form looks like y=mx+b. m is the slope of the line, b is the spot that the line will cross the y axis, also known as the "y intercept".
your two equations differ in their y intercept. The first will intercept the y axis at y=-2, the second at y=+6. they both have the same slope. Therefore, when you change from y=3x-2 to y=3x+6 the slopes stay the same but the line moves up 8 units.

I hope this helps. If you have questions please ask.