Let the subtracted algebraic expression may be B’ say
3x2 – 4y2 + 5xy – B = – x2 – y2 + 6xy + 20
B = (3x2 – 4y2 + 5xy) – (- x2 – y2 + 6xy + 20)
= 3x2 – 4y2 + 5xy + x2 +y2 – 6xy – 20
= (3x2 + x2) ( – 4y2 + y2) + (5xy – 6xy) – 20
∴ B = 4x2 – 3y2 – xy – 20
∴ The required subtracted expression is B
= 4x2 – 3y2 – xy – 20