Your original basis is your purchase price $51,000.
Then you pointed out having a mortgage balance $143,000 - that most likely because you refinanced - correct?
So we need to know how the loan proceeds were used?
If used for improvements - that amount is added to the basis, but whatever you used for personal or other reasons woudl not affect our calculations.
Assuming you used $20,000 on improvements - so your adjusted basis becomes $51,000 + $20,000 = $71,000.
Now - because the property was converted to rental - we need to know your accumulated depreciation.
Residential rentals are depreciated over 27.5 years.
So if the property was rented for 12 years - accumulated depreciation would be estimated as $71,000 / 27.5 * 12 = $30982
and your adjusted basis will be $71,000 - $30982 = $40018
Thus we may estimated your gain on the sale
$170,000(selling price) - $40018 (adjusted basis) - $10,000(selling expenses - estimated) = ~$120,000
That will be your taxable gain - and will be taxed as long term capital gain at reduced rate because the property was owned more than a year.
The rate will be up to 15% for most taxpayers - but may be 20% for high income persons.
There is no income tax in Florida
So rough estimation for your federal income tax liability based on assumptions above could be $120,000 * 15% = $18,000.