Answer:
The scale of the map is inch = 20 feet.
This statement must be incorrect or the fraction of the inch is missing.
Because 6" long, at a scale of one inch = 20 feet, would only be 120 feet x 120 feet.
120 x 120 = 14,000 ft^2 and each of the four quadrants wouls only be 3600 ft^2.
That's a 33% increase.
I calculated this using the formula:

Where n = the new value (16 in your question), o = the old value (12 in your question) and the result is outputted as a percent increase. You can check that this is correct by finding 33% of 12, adding the result to 12, and checking that the result equals your "new" number.
Note that 33% is only an approximation as your question requires a number rounded to the nearest whole.
2(b+3) = 2
2b + 6 = 2 ( -6 )
2b = -4 ( / 2)
b = -2