Amir's checking account charges a $9.75 monthly service fee and a $0.12 per-check fee. if amir writes 15 checks per month, should he switch to a checking account that charges an $11.50 monthly service fee and no per-check fee

Amir should switch because his monthly fee are currently greater than $11.50. So, if each check costs $0.12 cents, and he writes 15 a month, you multiply 0.12 by 15
0.12 times 15 equals $1.80
Additionally, you add $1.80 to $9.75, which comes out to $11.55
In summary, you will spend 5 cents more on the fist plan, than you would on the second.
thats why you should switch.

No because if you do the higher number minus the lower number you will get a positive. if you do it the other way around you will get a negative number