we know that
the length of rectangle is 5*(x+6) cm
the breadth of rectangle is (2x+6) cm
perimeter=192 cm
perimeter of a rectangle=2*[length+breadth]
so
192=2*[5*(x+6)+(2x+6)]----> 192/2=[5x+30+2x+6]---> 96=7x+36
7x=96-36-----> 7x=60--------> x=60/7 cm
the answer is
x=60/7 cm or 8.57 cm