<<VilGebra`

A=(({{-4x, 5x, -(2/3)x, (1/2)x}, {0, -(3/2)(x-1), x+2, 1}, {0, 0, -2x, 0}, {0, 0, 0, -(3/4)}}));
Determinante[A,fila->4]
Solve[Det[A]==9x,x]