x - 5.6 = -1.7
We are subtracting 5.6, and the to undo the subtraction we must do addition. So add 5.6 on both sides
x - 5.6 + 5.6 = -1.7 + 5.6
Simplify it
x = -1.7 + 5.6
This can also be rearranged to become
x = 5.6 - 1.7
Finally, do the subtraction and you get
x = 3.9