-35a^4b^3c/21a^2b^4d
=-5a²c/3bd

9xy^2+6xyz/3x^2y
=3xy(3y+2z)/3x²y
=(3y+2z)/x

49-x^2/2x+14
=(7+x)(7-x)/2(7+x)
=(7-x)/2


a^3+2a^2b+ab^2/a^2b-b^3
=a(a²+2ab+b²)/b(a²-b²)
=a(a+b)²/b(a+b)(a-b)
=a(a+b)/b(a-b)
=(a²+ab)/(ab-b²)