Note that angle QAB has measure 90 - x.

Also, cos(QAB) = QB/AB = QB/18, so QB = 18(cos(90-x)).

Similarly, cos(CBR) = BR/27, so BR = 27(cos(x)).

Note that QB + BR = 30 (the bottom part of your diagram).

So, 18(cos(90-x)) + 27(cos(x)) = 30.

Use the fact that the graph of cosine is the same as the graph of sine (just shifted) to convert cos(90-x) to sin(x).

Finally, divide everything by 3.