\(a.3x^2y+\left(-4\right)x^2y+6x^2y\)
\(x^2y\left(3-4+6\right)\)
\(5x^2y\)
\(b.\left(-7\right)xy+\left(-12-12\right)+10xy\)
\(xy\left(-7+10\right)-24\)
\(3xy-24\)
\(3\left(xy-8\right)\)
\(c.12xyz+8xyz+\left(-5\right)xyz\)
\(xyz\left(12+8-5\right)\)
\(15xyz\)