|If you are using the formula you posted, i.e. this one:|
then I don't see how it could be returning 900.
It can only return 2 things:
A dash when C3 <> Solutions
-90 when C3 = Solutions
When C3 = Solutions, Excel will perform the mathematical operation 10 - 100 which equals -90.
If you want 10-100 to appear in the cell, you must enclose the string in quotes so that Excel treats it as text.
=IF(C3="Solutions", "10-100", "-")
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