=the sum of two numbers is 12. the difference of the same two numbers is -4. what are the numbers?
a + b = 12
a - b = -4
in the first equation we put a alone on one side of the equal sign
a = 12 - b
and substitute this into the secon equation
(12-b) - b = -4
12 - 2b = -4
12 + 4 = 2b
16 = 2b
16/2 = b
8 = b
Now enter the value for b back into the first equation and solve for a
a = 12 - b
a = 12 -8
a = 4
check
a + b = 4 + 8 =12
a - b = 4 - 8 = -4
