Step-by-step explanation:
due to the 90° rotation around the point (0, 1) both lines are perpendicular (intersecting each other at 90°).
and perpendicular slopes are reciprocal with opposite signs :
a/b and -b/a
remember, the slope is the ratio
(y coordinate change / x coordinate change)
when going from one point on the line to another.
the slope of BC is B (4, 5) to C (1, 2)
x changes by -3 (from 4 to 1)
y changes by -3 (from 5 to 2)
the slope is
-3/-3 = 3/3 = 1/1 = 1
the slope of EF is E (5, -4) to (2, -1)
x changes by -3 (from 5 to 2)
y changes by +3 (from -4 to -1)
the slope is
+3/-3 = 1/-1 = -1/1 = -1
so, it matches the defined criteria for a perpendicular slope.