You can put this solution on YOUR website!
If the x-intercept is 4, you're right, b=12. If the y-intercept is 4, then b=4. Check the problem to make sure you copied it right. There could be a type-o in the book.
find b in y = mx + b?
Question: Put in y = mx + b form, then graph. x-intercept=4, slope=-3
OR if the question was:
Put in y=mx+b form, then graph. y-intercept=4, slope=-3
If you need help with the graph, let me know.