Question

calculate a

Answer 1

You got it.

Answer 2

Theorem of the bisectrix of the angle gives the equation\[\frac{ 4 }{ a } = \frac{ 6 }{ 9 }\] just solve it

Answer 3

For example u can start by inverting:
a/4 = 9/6 = 1.5
a/4 = 1.5 ==> soo a=?