Let a = i - j + 2k and b = i + 2j - 4k

a) Find 2a - 3b
This one was easy. It should be -i - 8j + 16k
b) Hence find a value of m such that a, b and c are linearly dependent, where c = mi + 6j - 12k.
I struggled to solve this one. Please help me by showing your working and explanation thx.

Answer is m = 3/4

+1 -1 +2

+1 +2 -4
+m +6 -12

det = 0

-24 +12 +4m -4m +24 -12 = 0

I get any old m