I had a question about practice question #3 of the Angles, Triangles, and Polygons chapter.

I had a question about practice question #3 of the Angles, Triangles, and Polygons chapter. I understand the logic behind why A)15 is the answer (Triangle Inequality Theorem), but I think that C)22 could also be a possible answer because the problem never specifies whether RS is one of the two congruent sides or the non-congruent side. If RS were the non-congruent side, and 22 was the perimeter, the remaining sides would each be 6.5, which are not integers. Is my reasoning correct?

In question #10 of the backsolving chapter…

In question #10 of the backsolving chapter : in the xy plane, a line containing the points (a, a^3) and (10,40) passes through the origin. Which of the following could be the value of a?

I found the explanation in the answer key to be too time-consuming if I were to solve the equation with backsolving. Can you explain how to solve this question algebraically instead?