The slope (m) is 1. The y-intercept is 3.
You can use the formula m=y2-y1/x2-x1
When you plug the coordinates in, you get: 3-0/0-(-3)
This leaves you with 3/3, aka 1.
The y-intercept is whatever coordinate on the line makes contact with the y-axis. In this case, it is coordinate (0,3) making the y-intercept 3.