For the give dimensions and given equation it cannot be solved. But for given dimensions the following equation can be solved.
To solve above A can be (3x4) , X is (3,3) abd B is (4x4).
If you want to solve the given equation:
The dimensions of A should be (4x3), X is (3x3) and B is (4X4)
Check the dimensions once for your quesion. Read about pinv function and it's properties.