Use benchmarks to estimate each sum or difference 17/18 -3/7=?

Benchmarks?????

I do not know what a bench mark is, but I see that the only common denominator here is...18 * 7 = 126...so, using form of one...

7*17/7*18 - 18*3/18*7

119/126 - 54/126

= 65/126 ------simplest form, so what are benchmarks????