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The expression is 

Using Commutative property of multiplication, the statement can be written as

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.

In the given expression there are no grouping symbols and powers.

First operation is multiplication.

                                     (Multiply: )

The value of expression is 15a.