I am F years old and my brother is B years older. What is the sum of our ages?

If you are F years old and your brother is B years older, then your brother is F+B years old. Add your age (F) and your brother's age (F+B) and you get F + (F+B) = 2F+B

The sum of your ages is 2F+B

where F and B are placeholders for numbers.