1. The problem gives a variable $Ra$ with a value of 4.5 kn. 2. Assuming $kn$ is a unit (for instance, kilonewtons in physics or kilonewtons force), the value of $Ra$ is simply $4.5$ kilo units of force. 3. Without additional context or questions, we can state that $Ra = 4.5 \; kn$ is a fixed value. Final answer: $Ra = 4.5 \; kn$