When we do multiplications of polynomials we have to multiply every term of the first expression with every term of the second expression, then:
[tex]\begin{gathered} (k+4)^2=(k+4)(k+4) \\ =k\cdot k+4k+4k+4\cdot4 \\ =k^2+8k+4^2 \\ =k^2+8k+16 \end{gathered}[/tex]
Now, the error in the expression given is that they did not do all the possible multiplications, they only squared the two terms.
