Divide both sides by 3.1 and find g = 20.9/3.1 ≈ 6.7 ≈ 7.
You can estimate this without a calculator by saying 20.9 ≈ 21 and 3.1≈3, then 21/3=7.
In general, be careful with rounding before a calculation, because the rounding errors can be enlarged, messing up the accuracy of your answer. It's best to round only at the end.