EXPLANATION:
When some sum is negative, it is usually included within a parenthesis to indicate the sum;
therefore in reality the exercise looks like this:
[tex]\begin{gathered} 550x+(-10x) \\ 550x-10x \\ 540x \\ \text{the answer is 540x} \end{gathered}[/tex]So adding a negative quantity is equal to subtracting a positive quantity of equal absolute value.
Thus the sum of 550x and -10x is equivalent to subtracting from 550 the absolute value of -10, which is 10.
