Alright so in order to simplify the following ,we firstly must know that when the base of the exponents are same then their power gets added while performing multiplication.
Thus,
for a) 3x^2y^4*2x^6y
- 3*2(x^(2+6))(y^(4+1)) = 6x^8y^5
for b) xz^3*4x^4z^5
- 4(x^(1+4))(z^(3+5)) = 4x^5z^8
for c) 4a^3b^2*3a^6b^5
- 4*3(a^(3+6))(b^(2+5)) = 12a^9b^7
for d) 6s^5t*s^4t^2
- 6(s^(5+4))(t^(1+2)) = 6s^9t^3