First, rewrite the expression as:
(-8/21 xx -7/16)(x^2 xx x)(y^3 xx y^2) =>
(-color(red)(cancel(color(black)(8)))/(color(blue)(cancel(color(black)(21))3)) xx -color(blue)(cancel(color(black)(7)))/(color(red)(cancel(color(black)(16)))2))(x^2 xx x)(y^3 xx y^2) =>
(-1/3 xx -1/2)(x^2 xx x)(y^3 xx y^2) =>
1/6(x^2 xx x)(y^3 xx y^2)
Next, use this rule for exponents to rewrite the x term:
a = a^color(red)(1)
1/6(x^2 xx x)(y^3 xx y^2) => 1/6(x^2 xx x^color(red)(1))(y^3 xx y^2)
Now, use this rule of exponents to simplify the x and y terms:
x^color(red)(a) * x^color(blue)(b) = x^(color(red)(a)+color(blue)(b))
1/6(x^color(red)(2) xx x^color(blue)(1))(y^color(red)(3) xx y^color(blue)(2)) =>
1/6x^(color(red)(2)+color(blue)(1))y^(color(red)(3)+color(blue)(2)) =>
1/6x^3y^5
Or
(x^3y^5)/6
