power equation

Set y to be 50 and then take logs of both sides.

 

50 = 21.665x^-0.264

 

ln(50) = ln(21.665) - 0.264lnx   using the properties of logs.

 

Rearrange:   lnx = (ln(21.665) - ln(50))/0.264

 

x = e^{(ln(21.665) - ln(50))/0.264}