We need to find the matrix of the same dimension as the two matrices given to us , in order to find the answer.
Lets call this matrix as:
[x1 x2 x3 x4]
now we need to perform the following subtraction to obtain B;
[x1 x2 x3 x4] - [2 -7 6 1] = [-9 5 1 -1]
so lets write now element by element (as the subtraction of matrices has to be performed):
x1 - 2 = -9 therefore x1 = -9 +2 = -7
x2 -(-7) = 5 therefore x2 +7 = 5 so x2 = 5 - 7 = -2
x3 - 6 = 1 therefore x3 = 1+ 6= 7
x4 -1 = -1 therefore x2= -1 +1 = 0