logarithm

Isolate the log on one side to get $\log_{36}{x} = -\frac{1}{2}.$

From there, we get $x^{-\frac{1}{2}} = 36.$

Hope this helps.

log₃₆  = 7-13 + 13/2

LOG₃₆ x    = 1/2

36^log₃₆x = 36^1/2

x = 6