How old was Lisa x years ago if she is now 13? Which expression represents this?

a = 13 - x

Step-by-step explanation:

Age = a

(Any other letter of substitution for age would be fine)

For example if x was 5:

a = 13 - 5

a = 8

For further study:

Research algebra on either website. (eg: Maths Is Fun, Khan Academy ...)

Answer:13-x = y