No problem. Also, define equation once makes it easier:
In[8]:= 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[8]= {{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}}
y = 249 - (1 - a)/2.86 (b 34.21)/a/(b - 0.03) (Exp[b x - 0.03 x] - 1)
Out[9]= 243 - (11.9615 (1 - a) b (-1 + E^(-0.03 x + b x)))/(
a (-0.03 + b))
In[10]:= FindFit[data, y, {a, b}, x]
Out[10]= {a -> 0.430225, b -> 1.81015}
In[11]:= Simplify[y /. %]
Out[11]= 259.108 - 16.1084 E^(1.78015 x)