=[(x+1)(x+4)][(x+2)(x+3)]-[(x^2+5x)+3][(x^2+5x)+7]
=[(x^2+5x)+4][(x^2+5x)+6]-[(x^2+5x)+3][(x^2+5x)+7]
=[(x^2+5x)^2+10(x^2+5x)+24]-[(x^2+5x)^2+10(x^2+5x)+21]
=(x^2+5x)^2+10(x^2+5x)+24-(x^2+5x)^2-10(x^2+5x)-21
=3