a gas has a volume of 6460 mL at 870 mm Hg and 77.2 C what is the volume of the gas at STP?

Initial: V₁ = 6460 mL, P₁ = 870 mmHg, T₁ = (273.2 + 77.2) K = 350.4 K
At STP: V₂ = ? mL, P₂ = 760 mmHg, T₂ = 273.2 K

Combined gas law: P₁V₁/T₁ = P₂V₂/T₂
Volume at STP, V₂ = V₁ × (P₁/P₂) × (T₂/T₁) = (6460 mL) × (870/760) × (273.2/350.4) = 5770 mL (to 3 sig. fig.)

GTB's answer is just fine, but there is no real need to convert any of the unit other than the temperatures. Since the question asks for a volume in L, you can convert the initial volume, or convert your answer from mL to L. Also, you need to know that STP means 1 atmosphere pressure (or 760 mm Hg) and 0 C (273.15 K). So:

P1V1/T1 = P2V2/T2
870 mm Hg (6460 mL) / 350.35 K = 760 mm Hg (V2) / 273.15 K
V2 = 5766 mL = 5.77 L

use the ideal gas law, remember n does not change so p1V1/T1 = p2V2/T2 or V2 = p1V1T2/p2. The original volume, pressure and temperatures are p1, V1, and T1 values; STP values are p2, T2, V2; convert volumes to liters, pressures to atmospheres (not mm Hg) and temperatures to degrees K.