Isn't it k^2 - 8k in the first one ?
You must associate these expressions with the formulas you should know :
(a-b)² = a² - 2ab + b²
(a+b)² = a² + 2ab + b²
So in each case, you'll got to find what is a. Then find 2ab. Hence, you got b, and you complete your expressions with b²