Standard form to y intercept

If you are going to graph them, you can use the slope-intercept form above.

You can also just find the intercepts:

7x + 3y = 21

    when x = 0, y = 7     --->   (0,7)

    when y = 0, x = 3     --->   (3,0)

    plot these two points and draw the line that connects them

-7x + 3y = 21

   when x = 0, y = 7      --->  (0,7)

   when y = 0, x = -3     --->   (-3,0)

   plot these two points and draw the line that connects them

At what point do these two lines intersect?

If you are supposed to change from standard form to y-form (which is slope-intercept form):

7x + 3y = 21

       3y = -7x + 21                 subtract 7x from both sides

         y = -(7/3)x + 3             divide by 3

-7x + 3y = 21

         3y = 7x + 21                add 7x to both sides

           y = (7/3)x + 7            divide by 3

But, I don't know what the instructions of the problem are. (And, I don't know why there is a comment about absolute value.)

Taking a shot into the dark:  no, they aren't perpendicular and no, they aren't parallel.

Can you post back with the rest of the instructions?

Ok, so it's talking about laser beams.

"Two additional lazer beams are added to the design. The path of the beams can be modeled by the linear equations 7x+3y=21 and -7x+3y+21. Graph these beams below."

umm... this is a werid question. there is mirror on the ground i think. i'll upload a photo of the graph. The bottom line is a mirror (sorry i have extremley bad handwriting+ tiny text.)http://i59.tinypic.com/2m0eix.jpg

here's the first question, too (sorry again for handwriting+backwards)http://i57.tinypic.com/mi1n9x.jpg

Basicly I can't use negitive numbers because the mirror will be reflecting all the lazers. if you want closer pictures of the questions i'll take them.