[1] (3y)(7y^2)(-y^4) = [2] (21y^3)(-y^4) = [3] -21y^7
[1] Original Problem,
[2] Multiplied (3y) and (7y^2) to get (21y^3) {Multiply Coefficients 3 and 7 and add exponents 1 and 2}
[3] Multiplied (21y^3) and (-y^4) {Multiply Coefficients 21 and -1 and add exponents 3 and 4} to get final answer:
-21y^7