The question asks to find the correct equation to determine $x$ in the triangle $\triangle RST$. Here’s the reasoning based on the diagram:

1. Understanding the Triangle: The triangle is isosceles, meaning two sides are equal, and thus the two base angles at $R$ and $S$ are also equal.

2. Angle Notation:

   • $\angle T = 3x$ degrees
   • Both $\angle R$ and $\angle S$ are each $x$ degrees, because they are base angles in an isosceles triangle.

3. Triangle Sum Property: The sum of the interior angles of a triangle is always $180^\circ$. Therefore, we can set up the equation: $3x + x + x = 180$ Simplifying this, we get: $5x = 180$

4. Answer Choice: The correct answer is: $3x + x + x = 180$

Let me know if you need further details or explanation!

Here are five related questions for practice:

1. How would you solve for $x$ after setting up the equation $5x = 180$?
2. What if one of the angles in an isosceles triangle was known; how would you find the other angles?
3. What is the general property of the sum of angles in any polygon, not just a triangle?
4. How would the equation change if $\angle T = 4x$ instead of $3x$?
5. What distinguishes an isosceles triangle from an equilateral triangle?

Tip: For any triangle, always remember that the sum of its interior angles is $180^\circ$.