Tell your teacher to learn some algebra
(a + b)² = a² + 2ab + b²
(a + b)² = (a + b)*(a + b)
= a*a + a*b + b*a + b*b
= a² + a*b + a*b + b²
= a² + 2ab + b²

Example:
(2 + 3)² = 5²
= 25

(2 + 3)² = (2 + 3)*(2 + 3)
= 2*2 + 2*3 + 2*3 + 3*3
= 4 + 6 + 6 + 9
= 25

(2 + 3)² = 4 + 12 + 9
= 25