Help with this math question and i will give you 40 points and mark you the brainliest

Answer:
a) CD = 6a + 4.5b
b) k = 4
Step-by-step explanation:
CD = CA + AB
CA = -AC
CD = -3b + 6a + 7.5b
CD = 6a + 4.5b
BC is parallel to CD
BC = BA + AC
BC = ka + 3b
BC is a scalar multiple of CD
Scale factor:
3/4.5 = 2/3
k = ⅔(6)
k = 4