Explanation:
the slope is a line is always the factor of x.
and it is the ratio (y coordinate difference / x coordinate difference) when going from one point to another.
y = 2x + 4
has the slope 2. which is 2/1. so, for every increase of x by 1 unit, y increases by 2 units.
and this line has a y-intercept of 4 (the y-value when x = 0).
remember, the slope-intercept form of a line is
y = ax + b
a is the slope, b is the y-intercept.
a perpendicular slope (90°) turns that ratio upside down and flips the sign (because if the original line was going up, the perpendicular line is going down and vice versa).
so, the slope of the perpendicular line is -1/2.
the y-intercept was already given to us, so the desired line equation is
y = -1/2 x + 3
or
y = -x/2 + 3