Y=70.19+(8.63*X)
To find the total she spent all you have to do is add everything together. The cost of the pants was $70.19, to find how much she spent on shirts you take the amount each shirt was, which in this case is $8.63, and multiply it by how many shirts she bought. We don't know how many shirts she bought so we use the varriable of X to represent that number.
(I used the asterisk * to indicate the multiplication)