A rational number is any number that can be expressed as a ratio of two integers (where the denominator is not zero).
- -5 can be expressed as -5/1, which is a ratio of two integers, so it is rational.
- \(\sqrt{4}\) is equal to 2, which can be expressed as the ratio 2/1, so it is rational.
- \(3\frac{3}{4}\) is equal to 15/4, which is a ratio of two integers, so it is rational.
- \(\sqrt{90}\) cannot be expressed as a ratio of two integers. It is irrational.
Therefore, the number that is not rational is \(\sqrt{90}\).