C=P[(1+r)^n-1]
C = COMPOUND INTEREST
P = PRINCIPAL
r = rate per period
n = number of periods

b - 10 = 7
Take 10 to the other side.

b = 17
Multiply by 4 to isolate b

b × 4 = 17 × 4
4 and 4 cancels out
b = 68
We can set up a system of equations:
x + y = 12
x - y = 4
Adding these two together:
2x = 16
x = 8
Substituting 8 in the first equation:
8 + y = 12
y = 4
Hope this helps!