100/(19+(15x-84):6)=4

100/[19 + (15x-84)/6] = 100/(19*6+15x-84)/6 = 100/(114+15x-84)/6 =100/(15x+30)/6 = 100*6/(15x+30) = 4600 = (15x+30)4600 = 60x + 120x = 480/60x = 8