Answer:
If you know what the formula y = mx +b is, then you know that mx is the slope and b is the y-intercept.
Let`s suppose the equation is y = 2x + 1. The line should be intersecting with the coordinate (0, 1) on it`s way across.
The y axis will ALWAYS be the vertical line. Whatever coordinate the line crosses on the y-axis is the y-intercept.
The y-intercept can also be a negative number.
Hope that helps. If you have any questions just comment them.