(m + n) ( m + n)
Multiply the first term in the first set of parentheses over the two terms in the second set
m * m + m * n
Multiply the second term in the first set of parentheses over the two terms in the second set
n * m + n * n
Combine the results
m*m + m*n + n*m + n*n =
m^2 + 2mn + n^2