30-60-90 triangle

If the base is 7 ,the height will be 7/√3 but when the base is 3 why we are doing multiplication? I’ve tried to understand the solution but can’t able to grasp it.

I’m extremely sorry for asking very basic questions.

Think about the general relationship of a 30-60-90 triangle (making it clear which angle is 30 degrees and which is 60).