\(a.5x-20y\)
\(\Leftrightarrow5\left(x-4y\right)\)
\(b.14x^2y-21xy^2+28x^2y^2\)
\(\Leftrightarrow7xy\left(2x-3y+4xy\right)\)
\(c.x\left(x+y\right)-5x-5y\)
\(\Leftrightarrow x\left(x+y\right)-5\left(x+y\right)\)
\(\Leftrightarrow\left(x-5\right)\left(x+y\right)\)
\(d.10x\left(x-y\right)-8y\left(y-x\right)\)
\(\Leftrightarrow10x\left(x-y\right)+8y\left(x-y\right)\)
\(\Leftrightarrow2\left(5x+4y\right)\left(x-y\right)\)