What should be added to -1 to get 10?

What number should be added to (-17) to get -19?

The tempareture and humdity of air are 27^(@)C and 50% on a particular day. Calculate the amount of vapour that should be added to 1 cubic metre of air to saturate it. The saturation vapour pressure is 3600Pa.

What is the unit vector parallel to vec a=3 hat i+4 hat j-2 hat k ? What vector should be added to vec a so that the resultant is the unit vector hat i ?

What should be added to 2x^(3) +6x^(2) -5x +8 to get 3x^(3) -2x^(2) +6x +15?