The greatest common factor is the largest factor that both terms share.
First, start with the coefficients, 14 and 28. The largest factor they both share is 14. 14 x 1 = 14, 14 x 2 = 28.
Next we move to the variables, abc and a²b²c³. The largest factor the both share is abc, all to the power of 1.
So, your GCF and final answer is 14abc