We compare triangles using the side lengths and angle measures. If a triangle has the same side lengths or an even side length ratio, they are similar. If a triangle has similar angle measures throughout, they are the same.
If we look at the question, we are given one triangle with angle measures of 40, 60, and 80 degrees. We are also given a second triangle with 2 of 3 side lengths; x, 60, and 80 degrees.
As we can see, both triangles have similar angle measures and note that a triangle must sum up to be a total of 180 degrees to form a complete triangle. If we add the angle measures of the first triangle, we get; 40 + 60 + 80 = 180.
We can create an equation to solve for the missing angle of the second triangle.
m∠X + 60 + 80 = 180
~Combine like terms
m∠X + 140 = 180
~Subtract 180 to both sides
m∠X = 40
Thus, we can see that the angle measures are the same, making both triangles similar.
Best of Luck!