You are using the if condition in the wrong way. You can perform you calculation first and then use array index to set the values of fb to 0 right after.
clear all
clc
P=93000 ;
L=45;
x= linspace(0,58,100);
sx=(P/L)*(x); %for scalar multiplication you don't need the .
sY=22;
n=0.53 ;
K=3.9 ;
fb=(sx.^-1).*((sx-sY)/K).^(1/n);
fb(sx<=sY)=0;
plot(x,fb,'-o')
