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.
1, the closest point of x = -3 is (-3,-6), which is one unit away from (-2,-6).
2x+6 = 20
take 6 from both sides
2x = 14
divide by 2 on both sides
x = 7
9+3=12
30+20=50
300+100=400
Then add.
12+50+400=462
<span>So 339+123=462</span>