of means multiplication
So
\(\frac{3}{4} \sf{of} \frac{16}{21} = \frac{3}{4} \times \frac{16}{21}\)
And then simplify the numerator ad denominator
= \(\frac{3 \times 16}{4 \times 21} = \frac{\ 1 \times 16}{4 \times 7}\)
And simplify the numerator and denominator again
= \(\frac{\ 1 \times 16}{4 \times 7} = \frac{\ 1 \times 4}{1 \times 7}\)
And we are finally left with the answer which is \(\frac{4}{7}\)
