The total amount of the resulting mixture can be calculated by adding up the volume of the given substances assuming that volume addition is applicable given the properties of the fluids used.
That is, T = 6 quarts + 10 quarts = 16 quarts
The total volume of the resulting mixture is 16.
Then, we do the component (antifreeze) balance by adding up the resulting antifreeze from the substances to the total. We let x be the percentage of antifreeze in the final mixture.
When I factor[ 2cos (square) - 5cos -3], I get (2cos + 1)(cos - 3). 2cos + 1 = 0, 2cos = -1, cos = -0.5,. Using inv cos on calculator, I get 120 degree related angle.