Given the radical expression:
#color(blue)((x sqrt 18) - (3 sqrt(8x^2))#
#rArr [x * sqrt(18)] - [3*sqrt(8)*sqrt(x^2)]#
#rArr [x * sqrt(9*2)] - [3*sqrt(4*2)*sqrt(x^2)]#
#rArr [x * sqrt(9)*sqrt(2)] - [3*sqrt(4)*sqrt(2)*sqrt(x^2)]#
#rArr [x * 3*sqrt(2)] - [3*2.sqrt(2)*x)]#
#rArr [x * 3*sqrt(2)] - [x*3*2*sqrt(2))]#
#rArr [3*x *sqrt(2)] - [6*x*sqrt(2))]#
#rArr 3x sqrt(2) - 6x sqrt(2)#
#rArr -3 x sqrt(2)#
Hence,
#color(red)((x sqrt 18) - (3 sqrt(8x^2))=(-3xsqrt(2))#