Step: Putting the value of m and n in required equation m=9x2−4xy+5y2
Then 2m=2[9x2−4xy+5y2]
=18x2−8xy+10y2 and n=−3x2+2xy−y2
⇒2m−n =18x2−8xy+10y2−(−3x2+2xy−y2) =18x2−8xy+10y2+3x2−2xy+y2 =18x2+3x2−8xy−2xy+10y2+y2 =21x2−10xy+11y2
Hence, the answer is 21x2−10xy+11y2.