SAT Question 5c94e6fa: Linear Equations In One Variable answer and explanation with Desmos solution

It's given that the equation $3x+21=3x+k$ has infinitely many solutions. If an equation in one variable has infinitely many solutions, then the equation is true for any value of the variable. Subtracting $3x$ from both sides of the given equation yields $k=21$. Since this equation must be true for any value of $x$, the value of $k$ is $21$.