Hello.

Start by assuming the opposite (that it is rational). Then there exist two integers a and b, whose greatest common divisor (GCD) is 1, such that a/b is the square root of three. That's what rational means.

Using this assumption, you should be able to show that both a and b are multiples of 3 and that means the GCD is at least 3, which is a contradiction of your assumption, meaning that square root of 3 is not rational.

Hope this helps,
Stephen La Rocque.