The basic solution system is the maximal linearly independent group of solutions. If a system of equations has a solution and is not unique, there are infinite solutions of maximal linearly independent groups and infinite basic solution systems. But each basic solution system contains the same number of vectors. So as long as the basic solution system is given, the rank is also given, and the number of ranks is equal to the number of vectors in the n- basic solution system. Therefore, the basic solution system given by the topic is only 1 vector, which shows that the rank of A is equal to 4- 1=3.