The expression 12a+7s is the cost, in dollars, for a adults s students to enter the local marching band competition. find the total cost for 4 adults and 30 students.
before you delete this i did try on my own and this has been confusing me

If 12A is equal to one adult, then this means that one adult will cost $12 to enter.
The same applies for the students, if 7s is equal to one student, then the cost of one student is $7.
As you've got four adults, then you need to find the cost of 4, and you do this by multiply 4 by 12.
4x12=$48
As you've got 30 students, you then need to multiply 30 by 7.
30x7=$210.
To find the total cost, you've then got to add $48 and $210 together.
$48+$210= $258.

Therefore, the total cost for 4 adults and 30 students is $258
Hope this helps :)