2a+3b=0,a=-3b/2
3a^2-5ab+2b^2/2a^2+3ab-5b^2
=(3a-2b)(a-b)/(2a+5b)(a-b)
=(3a-2b)/(2a+5b)
=(-9b/2-2b)/(-3b+5b)
=-13/4