please solve my problem

draw AB =4.5 cm (why 4.5.? because it's 3.5 +1)

Now on AB mark point C such that BC=1(or AC=3.5)

With AB as diameter , draw a semicircle.

At C, construct a perpendicular to AB , meeting the semicircle at D.

 Then CD is the required length which is equal to square root of AB.