3 6/7 =27/7; a = √(27/7) =√ ((3*9)/7) =3√(3/7) =3*√3 /√7
1 5/7 =12/7; b = √(12/7) = √(( 4*3)/7) =2√(3/7) =2*√3 /√7
175 = 7*25; c = √(3/175) =√3/(5*√7)
(a-b) : c = a/c - b/c
-----------------------------------------------------------------------
a/c = 3*√3 /√7 : [ √3/(5*√7) ]=3*5 =15
b/c = 2*√3/√7 : [√3/(5*√7) ] = 2*5 =10
a/c -b/c = 15 -10 =5
ответ 5