The expression is 

          (Replace y, z with 4, 12)

To simplify the expression following order of operations to performed:

1.   Evaluate the expressions inside grouping symbols.

2.   Evaluate Powers.

3.   Multiply and/or Divide in order from left to right.

4.   Add and/or Subtract in order from left to right.

According order of operations inside the grouping symbols should be evaluated first.

First operation is evaluating expression within the parentheses.

              (Multiply: )

Next operation is addition.

                                    (Add: )

Next operation is multiplication.

                                          (Multiply: )

Next operation is subtracting \ \

                                                   (Subtract: )

The value of the expression is 13.