Because the modules of A and B are both 3,4, the original formula can be 9-4a * b+64 = 36+12a * b+144, so vector A * vector b=- 107/ 16. Let the angle between a and b be y, cosy.
This kind of problem is relatively simple. Note that the squared flight is vector a* vector B, not simple a* B ... In addition, you need to know the angle between cos vector A and vector B = vector A * vector B/| A | | B |