It would be easier if you posted in <> format. You had some typos x*b needs to have a space in between x b and functions like Exp[] need square brackets [...]
In[6]:= data = {{0, 249}, {0, 243}, {0.13, 241}, {0.25, 237}, {0.25,
237}, {0.38, 229}, {0.5, 217}, {0.5, 217}, {0.63, 208}, {0.75,
198}, {0.75, 200}}
Out[6]= {{0, 249}, {0, 243}, {0.13, 241}, {0.25, 237}, {0.25,
237}, {0.38, 229}, {0.5, 217}, {0.5, 217}, {0.63, 208}, {0.75,
198}, {0.75, 200}}
In[7]:= fit =
FindFit[data,
249 - (1 - a)/
2.86 (b 34.21)/a/(b - 0.03) (Exp[b x - 0.03*x] - 1), {a, b}, x]
Out[7]= {a -> 0.170384, b -> 0.84151}
In[8]:= y =
243 - (1 - a)/2.86 (b 34.21)/a/(b - 0.03) (Exp[b x - 0.03*x] - 1) /.
fit
Out[8]= 243 - 60.3948 (-1 + E^(0.81151 x))