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