12 = 1 ten + 2 units.

12² is the same as (10 + 2)².

Let us square (10 + 2), that is, multiply 10 + 2 by 10 + 2.

10 + 2

10 + 2

---------------

10² + (10 × 2)

+ (10 × 2) + 2²

--------------------

10² + 2(10 × 2) + 2²