(5 × Level) + your Int + defenders Con

Interpreting equations like this isn't defined by the game rules and so we use the english meaning. Unfortunately, english (and most other languages AFAIK) isn't very good at it, which is a big reason for using mathematical symbolics. As a good rule; take the simplest reading as you scan through. This gives:

$$ 5 \times \text{Level} + \text{Your Int} + \text{Defender's Con} $$

which for anyone not fully comfortable with order of operations equals

$$ (5 \times \text{Level}) + \text{Your Int} + \text{Defender's Con} $$

If it were to mean

$$ 5 \times ( \text{Level} + \text{Your Int} + \text{Defender's Con} )$$

it should (in my opinion) been worded as (italics to emphasise change):

equal to five times the sum of your level in this class + your Intelligence modifier + the Iron Defender’s Constitution modifier

As additional support, the same problem of having such equations in D&D rules have appeared for Arcane Ward where lead designer Jeremy Crawford has chimed in on twitter (not official ruling, but gives what the intended meaning is) with the statement that

The ward has hit points equal to twice your wizard level + your Intelligence modifier.

should be read as:

its hit point maximum equals your Intelligence modifier plus twice your wizard level.

Meaning to be read as

$$ (2 \times \text{Level}) + \text{Int} $$

similar to that above.

As an addendum: the final version of the subclass clarifies the calculation as:

equal the steel defender’s Constitution modifier + your Intelligence modifier + five times your level in this class

and this is how the feature is implemented in D&D Beyond. (Thanks to @Garret Rooney and @V2Blast)