What do we get after expanding (p + 3q - 2z)²?

The correct answer is: p²  +  9q²  +  4z²  +  6pq – 12qz – 4zp

We know that (a  +  b  +  c)² = a²  +  b²  +  c²  +  2ab  +  2bc  +  2ca

(p + 3q - 2z)²can also be written as (p + 3q + ( - 2z))².

Here, a = p, b = 3q and c =  - 2z

Therefore, using that formula we get (p + 3q + ( - 2z))²
= p²  +  (3q)²  +  ( - 2z)²  +  2(p)(3q)  +  2(3q)( - 2z)  +  2( - 2z)(p)

= p²  +  9q²  +  4z²  +  6pq – 12qz – 4zp.