Which one of these lines uses iambic parimeter

ambic pentameter is fives sets of iambs, or one unstressed followed by one stressed.

The only choice that matches that description is option A.  

The others all start with a stressed syllable, so they can not be iambs.

Explanation:

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substitute y = 2x - 4 to x + 2y = 12:

x + 2(2x - 4) = 12     use distributive property

x + (2)(2x) + () = 12

x + 4x - 8 = 12     add 8 to both sides

5x = 20     divide both sides by 5

x = 4

put the value of x to the first equation:

y = 2(4) - 4

y = 8 - 4

y = 4