He uses the slope of 2

Answer:
y = 2x + 3
Step-by-step explanation:
The equation of a line in slope- intercept form is
y = mx + c ( m is the slope and c the y- intercept )
here m = 2
The line passes through the y- axis at (0, 3) ⇒ c = 3
y = 2x + 3 ← equation of line