（0.3-x）（500+10x)=120
0.3*500+0.3*10x-500x-10x²=120
150+3x-500x-10x²=120
-10x²-497x+150=120
-10x²-497x+30=0
10x+497x-30=0
a=10,b=497,c=-30 ······