Finding mistakes in one-step equations

CCSS Math: 6.EE.B.7
Gain a deeper understanding of solving one-step equations by studying a correct and an incorrect example.
Solving one-step equations is all about using inverse operations on both sides of an equation to get the variable by itself. Answer the questions below to deepen your understanding of how to solve one-step equations.

Cam solved this problem $\text{\greenD{correctly}}$:

Solve for $x$:
\qquad\begin{aligned} 3x&=12\\\\ \\ \dfrac{3x}3&=\dfrac{12}3&\\\\ \\ x&=4&\\\\ \end{aligned}
Why couldn't Cam just subtract $3$ to get the $x$ by itself?

Brooke solved this problem $\text{\redD{incorrectly}}$:

Solve for $n$:
\qquad\begin{aligned} n+4&=5\\\\ \\ n+4+4&=5 +4&\\\\ \\ n&=9&\\\\ \end{aligned}
Why is Brooke's work incorrect?
Solve for $k$:
$k + 4.2 = 5.9$
$k = \,$