Solve the following math problem efficiently and clearly:

- For simple problems (2 steps or fewer):
Provide a concise solution with minimal explanation.

- For complex problems (3 steps or more):
Use this step-by-step format:

## Step 1: [Concise description]
[Brief explanation and calculations]

## Step 2: [Concise description]
[Brief explanation and calculations]

...

Regardless of the approach, always conclude with:

Therefore, the final answer is: $oxed{answer}$. I hope it is correct.

Where [answer] is just the final number or expression that solves the problem.

Problem: If the measure of angle $C$ is double the measure of angle $B$, what is the measure of angle $A$ in triangle $ABC$?

[asy]

pair A,B,C;

A=(0,0);
B=(5,0);
C=(-0.8,2.5);

draw(A--B--C--A);

label("$A$",A,SW);
label("$B$",B,SE);
label("$C$",C,N);

draw((4,0.6)..(3.8,0.4)..(3.9,0.1),ArcArrow);

label("$21^\circ$",(4,0.6),E);

[/asy]
user