The midpint is just (the average of the 2 x values, the average of the 2 y values)
To get the average of 2 numbers you add them together and divide by 2
So the average of 12 and 5 for instance is (12+5)/2 = 17/2 = 8.5
If you want the more formal look,
The mid point of
\((x_1,y_1)\;\;and\;\;(x_2,y_2) \;\;is\;\;\;\;(\frac{x_1+x_2}{2},\frac{y_1+y_2}{2})\)
The y intercept is where the line crosses the y axis and you can see that it crosses at 4
SO the y intercept, which is always given the letter b =4