A friend of mine let me borrow one of those MFJ antenna analizier

Now if I have a reading of 40 R and 35 X then if I put a cap across the coax
with an impedance of 35 ohm it should cancel out the inductive reactance and
I
should only see the 40 ohm resistive?

The MFJ analyzer does not give the sign of the jX component of the impedance.
If the jX component is 35 ohms inductive reactance, then 35 ohms capacitive
reactance will cancel. The instruction manual says it's up to you to figure
the sign of jX.
In general, if you increase frequency and jX decreases, jX is capacitive.
Converse, inducutive.
73 Gary N4AST