4x^2-3y^2=7,等式两边同乘以3,
即得12x^2-9y^2=21 ① ,
同理：3x^2+2y^2=19,等式两边同乘以2,
即得6x^2+4y^2=38 ②,
①+②即得18x^2-5y^2=59