For the equation:
Identify the slope and the y-intercept by comparing with the general equation of a line with slope m and y-intercept b:
The slope of the given equation is equal to 6, and the y-intercept is equal to 4.
Both Mia and Mark agree that the y-intercept is equal to 4, but the slopes of the lines are different.
Notice that the red line has a negative slope, so, by no means that would be the graph of y=4+6x.
Observe that for a change of 1 step over the X-axis, the dotted line has an increase of 6 units over the Y-axis. You can look at the values for x=-1 and x=0. Therefore, the dotted line has a slope of 6 and a y-intercept equal to 4, which are precisely the values given by the equation.
Therefore, Mia is right.