Let $\angle A$ and $\angle B$ both be $x$ degrees. How can you write each angle of the triangle in terms of $x$?

The three interior angles of any triangle add up to $180$ degrees. Write an equation involving $x$, $x$, and $72$.

After you combine like terms, you should get an equation of the form $2x + 72 = 180$. Solve this equation to find $x$, the measure of $\angle A$.

Enter the linear equation y = 2x + 72 to represent the sum of the two equal angles and the $72^\circ$ angle, and enter the horizontal line y = 180 to represent the total angle sum of a triangle.

Use the intersection tool (or tap/click where the graphs intersect) to find the point where the two graphs cross; the x-coordinate of this intersection is the measure of $\angle A$ in degrees.

In $\triangle ABC$, we are told that $\angle A$ and $\angle B$ are congruent (equal in measure).

Let the common measure be $x$ degrees. Then:

• $\angle A = x$
• $\angle B = x$
• $\angle C = 72$ degrees (given).

The sum of the interior angles in any triangle is $180$ degrees.

So we write the equation:

Combine like terms:

Now isolate $2x$:

From the previous step we have:

Divide both sides by $2$:

So $\angle A$ measures $54$ degrees, which corresponds to choice B.