(3,1/7х+5,1/2)*1,7/11=21
( 3,1/7х + 5,1/2 ) = 21 : 1,7/11 3,1/7х + 5,1/2 = 21 : 18/11 3,1/7х = 21 : 18 * 11 - 5,1/2 22/7х = 77/6 - 11/2 22/7х = (77 - 33)/6 22/7х = 44/6 = 22/3 х = 22/3 : 22/7 х = 7/3 х = 2,1/3