Let us assume that 2/√7 is a rational number.
2/√7 x √7/√7 = 2√7/7 is a rational number
Which is only possible if 2/7 is rational and √7 is rational.
But the fact is √7 is an irrational.
Which is contradicts to our assumption.
2/√7 is an irrational number. Hence proved.