(5.7a^2-2.1ab+b^2)-(3.9ab-0.3a^2+2b^2)
если a=-1 b=5

(5,7а^2-2,1аb+b^2)-(3,9ab-0,3a^2+2b^2)=5,7a^2-2,1ab+b^2-3,9ab+0,3a^2-2b^2=6a^2-6ab-b^2=6*(-1)^2-6*(-1)*5-5^2=6+30-25=11