Question 157733
Let's plot the points.
They should all line up on a line.
{{{drawing( 300, 300, -2, 14, -10, 10,grid(1),
circle( 3, 8, .2 ),
circle( 0, 4, .2 ),
circle( 12, -3, .2 ),
green(line( 3, 8, 0, 4)),
green(line(3,8,12,-3)))}}}
.
.
.
No that doesn't look like a good solution.
When you look at the graph you see that when you move from x=0 to x=3, y moves from y=4 to y=8. 
A linear graph will maintain that relationship(add 3 to x, add 4 to y). 
So when you move from x=3 to x=6, then y would move from y=8 to y=12.
{{{drawing( 300, 300, -2, 14, -2, 14,grid(1),
circle( 3, 8, .2 ),
circle( 0, 4, .2 ),
circle(6, 12, .2 ),
green(line( 3, 8, 0, 4)),
green(line(3,8,6,12)))}}}
Keep going in this manner and eventually you'll reach x=12 and you'll have your y. 
Please re-post if you get stuck.
Good luck.