Answer:
3s^2 +9st +6t^2
Explanation:
Each term of one factor is multiplied by all terms of the other factor and like terms are combined. This is an application of the distributive property.
= 3s(s +t) +6t(s +t)
= 3s^2 +3st +6st +6t^2
= 3s^2 +9st +6t^2