x = -0.89
Explanation:
when subtracting a positive number to a negative it will always be negative or atleast 0. It is like a number line, when subtracting on a number line, you always go left, which is also why its negative.
How I solved it: (just another way to solve this)
9.10-8.20=0.89
since the 9.10 was actually a negative(-9.10), it would make 0.89 negative, making it -0.89.