Algebra/ˈældʒɪbrə/ Definition 1 of 2Pro Tip 1/3Not Just Numbers Algebra uses symbols, often letters, to stand in for unknown numbers in equations. Find the value of 'x' in the equation x + 5 = 9. Pro Tip 2/3Problem Solving Algebra is often about finding the missing numbers that make an equation true. If 3a = 9, what does 'a' equal? Pro Tip 3/3Real-World Use Used for calculating things in real life, like expenses, distances, or time. Price of 5 apples (a) is $10: 5a = 10.