ab=5(a+b)+75
a(b-5)=5b+75
a=(5b+75)/(b-5)
=5+100/(b-5)
ab=5b+100+500/(b-5)
dab/db=5-500/(b-5)^2=0
b-5=10
b=15
ab=75+150=225
b=15
15^2=150+75=225