Question

Shannon sells handmade wallets for $12 each.

Answer 1

To answer the question above, 
Let n be the no. of sold wallet
1 wallet = $12
yesterday = n
today = n - 3
over the past two days = $396

an equation that models the situation: n x $12 + (n - 3)  x $12 = $396
n x 12 + (n - 3) x 12 = 396
n x 12 + n x 12 - 3 * 12 = 396
2 x n x 12 - 36 = 396
n x 24 = 396 + 36
n x 24 = 432
n = 432 / 24

The answer is
Shannon sold n = 18 wallets yesterday.

Answer 2

I hope this helps you