a*b=375
(a+5)*(b-5)=400
ab-5a+5b-25=400
ab-5a+5b=425
375+5(a-b)=425
5(a-b)=425-375
5(a-b)=50
a-b=50:5
a-b=10
a=10+b
(10+b+5)(b-5)=400
15b^2-75b=400
(10+b)b=375
10b+b^2=375
15b^2-75b=400 | *(+2)
{
b^2+10b=375 | *15
30b^2-150b=800
{
15b^2+150b=5625
35b^2=6425