Explanation:
if the line equation is in the form
y = ...
the slope is always the factor of x.
x + y = 4
y = -x + 4
so, the slope is -1.
a parallel line has the same slope.
when having a surviving point we can use the point-slope form as equation :
y - y1 = m(x - x1)
with m being the slope, and (x1, y1) being a point on the line.
so,
y - 3 = -1(x - 2)
simplified we get
y - 3 = -x + 2
y = -x + 5
that would be the slope-intercept form (+5 being the interception point on the y-axis).