题目

The class was right in the middle of a math problem. Suddenly, Joe laughed out loud at the back of the classroom. Everyone stopped and looked at him.“What are you laughing at?” Mr. Steinman.“Nothing.” Joe answered.“Joe. Office. Now,” Mr. Steinman became angry.“Fine, I’d rather be there anyway.” Joe said.“I’ll be calling home after school, too,” Mr. Steinman said.“Ask my mom to buy pizza for dinner when you call,” Joe suggested.His teacher opened his mouth to reply, but thought better of it. Instead, he continued the maths lesson. Joe took his time to get to the door解方程组：（1）x-y=3y+2x=15（2）3(x+y)-4(x-y)=4x+2y7=2