(4a-1)^2-25=4^2a^2-2*4a*1+1^2-25=16a^2-8a+1-25=16a^2-8a-24
9y^2-(1+7y)^2=9y^2-(1^2+2*1*7y+7^2y^2)=9y^2-(1+14y+49y^2)=9y^2-1-14y-49y^2=-1-14y-40y^2
(a+11)^2-121=a^2+2*a*11+11^2-121=a^2+22a+121-121=a^2+22a
(m+n)^2-(m-n)^2=(m^2+2mn+n^2)-(m^2-2mn+n^2)=m^2+2mn+n^2-m^2+2mn-n^2=4mn
(4c-x)^2-(2c+3x)^2=(4^2c^2-2*4cx+x^2)-(2^2c^2+2*2c*3x+3^2x^2)=16c^2-8cx+x^2-4c^2-12cx-x^2=12c^2-4cx