I need to find the resultant vector c in magnitude and direction.

I found the magnitude to be 7.2 using cosine rule, i.e.:

However, I'm not able to find the magnitude.

So far, I have tried the following:

Since resultant vector c = a + b

ax = -(6 cos 40)

ay = 6

bx = 3 cos 70

by = 3

Then,

cx = ax + bx

cy = ay + by

But that doesn't seem to help. (Thinking)