Hints:
* Suppose $B$ was the final price of the blouse and $J$ the final price of the jeans.
* Can you translate "she paid $\$12$ more for the blouse than for the jeans" into an equation involving $B$ and $J$?
* Can you translate "she spent a total of $\$84$" into an equation involving $B$ and $J$?
* Can you solve these to find $J$?
* Suppose $j$ was the original price of the jeans.
* Can you translate "Marcy bought one pair of jeans at 70% off" into an equation involving $j$ and $J$?
* Can you solve this to find $j$?