NCERT Example - Identity 1 - We Distribute, Yet Things Multiply
Question: Expand (a + m)(b + n). Use it to find how the product changes when one number decreases by 2 and the other increases by 3; and when both decrease (by 3 and by 4).
Answer: (a+m)(b+n)=ab+an+mb+mn; (a-2)(b+3)=ab+3a-2b-6; (a-3)(b-4)=ab-4a-3b+12.
Full step-by-step NCERT solution with diagrams on Something by Zero.