Step-by-step explanation: To subtract the integers -15 - (-12), it's important to understand that when we have minus a negative, we can change it to plus a positive so -15 - (-12) becomes -15 + (+12).
So starting at 0 on our number line, -15 moves us 15 units to the left and +12 moves us 12 units back to the right and we end up at -3.
Since (density = mass/volume), we can get the mass/weight of the liquid by integrating the density over the interior of the tank. This is done with the integral
which is more readily computed in cylindrical coordinates as