Time (min) | 5 | 10 | 15 | 20 | 25 | 
Charge ($) | 8 | 13 | 18 | 23 | 28 |

I'll show you three ways to find it.

Plot the 5 points on a graph:



Draw a line through them



Now you can see that the green line crosses the y-axis at 3,

That means the y-intercept is the point (0,3).

Another way to do it is to look at your list:

Time (min) | 5 | 10 | 15 | 20 | 25 | 
Charge ($) | 8 | 13 | 18 | 23 | 28 |

Notice that the times are spaced out in steps of 5 minutes
starting with 5

Also notice that the charges are spaced out is steps of 5,
starting with 8.

Now suppose we wanted to put in a point before (5,8). It 
would have to be (0,3)

Time (min) | 0 | 5 | 10 | 15 | 20 | 25 | 
Charge.($) | 3 | 8 | 13 | 18 | 23 | 28 |

That's because the point before the first one would have to be
gotten by subtracting 5 from 5 getting 0 and 5 from 8 getting 3.
So the y-intercept is (0,3)

You can also do it by first finding the slope using the first two
points, (5,8) and (10,13) and then using the point-slope form.











Now the y-intercept is gotten by substituting x=0





So the y-intercept is (0,3).

Edwin