5ab-9/4a^2b-9/4ab+1/2a^2b
=(5ab-9/4ab)-(9/4a^2b-1/2a^2b)
=11/4ab-7/4a^2b
=1/4ab(11-7a)
=1/4*1*(-2)*(11+14)
=-25/2
=-12.5