Explanation:
so, we have
6x² + 8xy + 8y²
and
5x² + 3xy + 16y²
we can combine only terms of the same kind (defined by the type and exponent of the used variables).
this is like adding fruit.
e.g. when we have a mixture of 3 apples, 4 oranges and 5 plums, and we add a mixture of 6 apples, 2 oranges and 3 plums - what do we get ?
we get a new mixture of
3+6 = 9 apples
4+2 = 6 oranges
5+3 = 8 plums
and exactly the same thing happens with mixing terms when adding them up.
so, we get
6+5 = 11x²
8+3 = 11xy
8+16 = 24y²
so, the result is
11x² + 11xy + 24y²
it is really that simple.