If the vector a and B satisfy | a | = 2, | B | = 1, and a &; (a + b) = 3, then a, B =?

If the vector a and B satisfy | a | = 2, | B | = 1, and a &; (a + b) = 3, then a, B =?

solution
a(a+b)=3
a²+ab=3
That is 4 + AB = 3
∴ab=-1
And ab = / A / / B / cos
∴/a//b/cos=-1
∴cos=-1/(2×1)=-1/2
∵∈[0,π]
∴=120