a) Use Gaussian elimination to find the values of a for the...

a) Use Gaussian elimination to find the values of a for the following equation system to have a solution:

2x + y − 2z = a + 7

3x − y + az = −3

−x + ay − 4z = 8 

b) Suppose that A, B and C are three 3 × 3 matrices for which A and B have inverses. Use the associative and distributive laws of matrix multiplication in order to solve the equation A^-1(A + X)B = C for X.

c) Find |1 3 4| (3*3 matrix) by cofactor expansion along the first row. 

| 2 2 1 |

| 3 −3 −9|

Thank you so much!