We are given the following statement.
Two angles are supplementary if they are a linear pair.
We are asked to write the above statement as an if-then statement.
In an if-then statement, we first write the "hypothesis" and then write the "conclusion"
If (hypothesis) then (conclusion)
For the given case,
Hypothesis = two angles are a linear pair
Conclusion = they are supplementary
So, the if-then statement is
If two angles are a linear pair, then they are supplementary.