Do you know how to add vectors? Break them into x- and y-components and add the components, then convert back to magnitude and direction. So - what are the x- and y- components of vectors a, b, and c? What do get for the sum of all three vectors? Finally - what is the magnitude of ths sum of all three vectors?
That's probably the simplest thing to do. But you could also use the "cosine law".
It should be clear that $\displaystyle \vec{a}+ \vec{c}$ has length 10- 2= 8 and is in the direction of c.
Now, adding $\displaystyle \vec{b}$ to that, we have a triangle with sides of length 8 and 3 and angle 180- 120= 60 between them. The length, x, of the third side of that triangle satisifies $\displaystyle x^2= 3^2+ 8^2- 2(3)(8)cos(60)= 9+ 64- 48(1/2)= 49$ so x= 7. The angle, A, that side makes with $\displaystyle \vec{a}$ is given by the sin law: $\displaystyle \frac{sin(A)}{3}= \frac{sin(60)}{7}$.