In a 30-60-90 triangle howdo you do the pathagerum therum

The same way you always do it.

a^2 + b^2 = c^2
In this case, if the smallest side is a,
a = c/2, because of the 30 degree angle.
c^2/4 + b^2 = c^2
b^2 = (3/4)c^2
b = c (sqrt3)/2