For 2017 we will file married jointly will earn less than $75000. We have $170000 in long term capital gains on investment property. What percent will we have to pay?

Married filing jointly with no dependents and using standard deduction means you have $55K of taxable income. That means the first $20,000 of capital gain will be taxed at 0%. The rest will be taxed at 15%.

It also depends on what you mean by investment property. If you claimed depreciation, over the years, the depreciation "recapture" is taxed at ordinary income.

For a more accurate estimate, you can try this tool https://turbotax.intuit.com/tax-tools/calculators/taxcaster/?s=1. Enter your regular income first to see the regular tax. Then add the sale to see the effect.
Enter the difference between the sale price and what you paid for it originally as a long term capital gain (LTCG). Enter the depreciation you've taken over the years (depreciation "recapture") as other income. Depending on how much total income you have LTCG are partially taxed at 0%, 15%, 20% and/or 23.8%. Depreciation recapture is taxed at your marginal rate, but not more than 25%.