Consider the following multistep word problem: The sum of the ages of a brother and sister is 28. If four times the brother's age is subtracted from three times the sister's age, the difference is 21.
Consider just the first sentence of this multistep word problem.
Sentence 1: The sum of the ages of a brother and sister is 28.
Give an equation that represents this statement using b as the age of the brother and s as the age of the sister.
Express your answer in terms of the symbols b and s.