Since all the sides add up to 10x+2, that means that 10x-2-<side1>-<side2>=<side3>. Plugging it in, we have 10x-2-5x-(2x+9)=side3, and 5x-2-(2x+9)=side3, then expanding it to 5x-2-2x-9=3x-11=side3
Explanation: ASA stands for "angle side angle". To use ASA, we must have two pairs of congruent angles, as well as a pair of congruent sides. The sides must be between the angles in question