Compute the value of the function

["sumaia.saad-eddin" <sumaia.saad-eddin@math.univ-lille1.fr>]

Good morning

I have the function f(x,a,b), I would like to compute the value of this 
function under the condition ((x-1)*b  \leq a \leq  2*x*b),
I write

if(a>2*x*b, print("a<=2*x*b is not required, [a,b,x] = ",[a,b,x]),);

My problem is that I do not know how can I add  the second condition 
((x-1)*b  \leq a )

Thanks for any help.
Sumaia