Which systems below would be most easily solved using the substitution method?

a. 5x + 7y = 45
2x - 7y = -14

b. 2m - 3n = -2
m + 4n = 24

c. 2x = -3y + 4
x = 2y - 5

d. 7x + 2y = 16
-21x - 6y =24

Which statement explains how you could use coordinate geometry to prove the opposite sides of a quadrilateral are congruent? a. use the slope formula to prove the slopes of the opposite sides are the same. b. use the slope formula to prove the slopes of the opposite sides are opposite reciprocals. c. use the distance formula to prove the lengths of the opposite sides are the same. d. use the distance formula to prove the midpoints of the opposite sides are the same.