so on a graph you have two axis's: the x (horizontal) and y (vertical). The y intercept is the point at which your line touches the y axis which is also when x=0. So your y intercept will always be (0,y) in this case when x is t and when t is 0, C (or y) is 8. x is your input (independent) and y is your output (dependent). The y intercept in the above model can be written as (0,8) or m=8
