What is the answer to this : expand y(y + 10)

When expanding brackets, you have to multiply whatever is outside by everything on the inside
The calculations you would do are
y x y
y x 10

Whenever a value is multiplied by itself once, we describe it as the value squared (²)
y x y can be written as y²

y x 10 is the same as 10 lots of y
In algebra, we don't write multiplication signs as it can get confusing so we write the number directly in front of the letter    10y

When put together, y(y + 10) expanded is

y² + 10y