Adding a negative number is the same as subtracting that number:
\[ -3 + x = x-3 \]
Subtracting a negative number is the same as adding that number:
\[ x - (-7) = x+7 \]
If you have an equation where you are adding and subtracting terms, you can move those terms around as long as you keep the sign of each term with the value, like this: \[ \begin{align} -3-(-6) + 4 &= -3+6+4\\ &=6+4-3\\ &=6-3+4 \end{align} \]
Hereโs a more complicated example where we move terms around to group together things with the same numbers and unknowns:
\[ \begin{align} \text{our initial expression:} \qquad& -x+3y-(-4x) -9+(-3y) -1\\ \text{rewrite the bracketed terms:}\qquad& -x + 3y + 4x -9 -3y-1\\ \text{group things together:} \qquad& 4x -x + 3y -3y -9-1\\ \text{simplify:} \qquad& 3x -10\\ \end{align} \]