The "Rule of 2 and 4" (Phil Gordon) says that you count your outs, in this case 7. Your chances of hitting an out on the turn are "outs x 2", and your chances of hitting an out by the river are "outs x 4". Both estimates are slightly off and a bit conservative.

With 7 outs, you'd estimate a 14% chance of hitting an out on the turn. Just a bit more than 6 to 1 odds needed for a call.

The EXACT probability is 7/47 = 14.89% or 5.7 to 1 odds. So if you use the Rule of 2 and 4, you'll always be a bit toward the safe side, as long as you insure that you're actually getting the odds you've calculated from the pot (+any implied odds).