The intercepts occur when both y=0 and x=0. So, setting y=0 gives you 0=3x+9. Now we simply solve for x:
Subtract 9 from both sides: -9 = 3x
Divide by 3 on both sides: -9/3 = x
Simplify: x=-3.
Now set x=0:
y=3*0 + 9
So y=9.
The intercepts occur at (0,9) and (-3,0)
