A 120 ml flask contains 2.7 micrograms of O2 at 17 degrees celsius. What is the pressure in the flask?

You use the ideal gas equation:
PV = nRT
Then you sort out the data:
P = pressure - what you want
V = volume = 120mL = 0.120L
n = mol O2
Calculate: Molar mass O2 = 32g/mol
Mol O2 in 2.7µg = 2.7*10^-6g
Mol = 2.7*10^-6 / 32.0 = 8.4*10^-8 mol O2
R = constant = 0.082057
T = temperture = 17°C = 290K
Substitute into gas equation:
P * 0.120 = (8.4*10-8) * 0.082057 * 290
P = (8.4*10-8) * 0.082057 * 290 / 0.120
P = 1.67*10^-5 atm
You do not specify what units you require for the pressure. Can you do the necessary conversion from atm to what you want .

For the oxygen gas in the flask :
Volume, V = 120 ml = 0.12 L
Mass, m = 2.7 μg = 2.7 × 10⁻⁶ g
Molar mass, M = 16.0 × 2 g/mol = 32.0 g/mol
Temperature, T = (273 + 17) K = 290 K
Gas constant, R = 0.08206 L atm / (mol K)

PV = nRT and n = m/M
Then, PV = (m/M)RT

Pressure, P
= mRT/(MV)
= (2.7 × 10⁻⁶) × 0.08206 × 290 / (32.0 × 0.12) atm
= 1.673 × 10⁻⁵ atm
≈ 1.7 × 10⁻⁵ atm (to 2 sig. fig.)