The cutoff frequency \(f_c\) for a first-order low-pass filter is defined as:\[f_c = \frac{1}{2 \pi R_1 C_1}\]Using the provided component values, the calculation is:\[f_c = \frac{1}{2 \pi \times 1.2 \times 10^3 \times 0.02 \times 10^{-6}} \approx 2.63 \, \text{kHz}\]
Final Answer: \[\boxed{2.63 \, \text{kHz}}\]