Answer:
Distributive property says that:
(A + B)*C = A*C + B*C
Now let's try to use it in our expression:
3 + 5*c*(2 + 6*c)
Here we can take the two terms inside the parentheses as A and B, and the term that multiplies them as C, then distributing we get:
3 + (5*c)*2 + (5*c)*(6*c)
Now remember that the multiplications are associative and commutative, then we can write this as:
3 + (2*5)*c + (5*6)*(c*c)
3 + 10*c + 30*c^2
And we can't simplify it anymore.