Evaluate as the limit of a sum.
Comparing
Now
where n h = b - a
Put a = 1, b = 2
Comparing we get
f(x) = 2x +3 , a = 1, b = 3
f(a+2h) = f(1+2h) = 2(1+2h)+3 = 5+4h
... ... ... ... ... ...
Now,