(x+6)(x-7)=14
x²+6x-7x-42=14
x²-x-56=0
(x-8)(x+7)=0
解得x=8或x=-7
7x(3-x)=4(x-3)
21x-7x²=4x-12
7x²-17x-12=0
(7x+4)(x-3)=0
解得x=-4/7或x=3