S=a+b+c
solve for b
Completely forgot how to do this

b = S - a - c

Step-by-step explanation:

You just move every other term except for the one you're solving for to the other side, using the opposite operations

S = a + b + c

S - a = b + c

S - a - c = b

b = S - a - c

OR

S = a + b + c

-b + S = a + c

-b = -S + a + c

b = S - a - c (we flipped all signs here to make b positive, like it's supposed to be. The justification for this is multiplying both sides by -1)

the monthly balance will be $75,000