64-4ab-27=6 1

4ab=-24

ab=-6

That is, the modulus of a times the modulus of b and then times the cosine of the included angle =-6.

So the cosine of the included angle is equal to-1/2.

The included angle is equal to 120 degrees.

|a+b|^2=a^2+2ab+b^2= 16- 12+9= 13

So |a+b|=√ 13