When you want to know the mean of
quantities, denoted by
, you use the following expresion

where
are the quantities.
In our case the mean is 130, i.e.
and the quantities are 104, 137, 154, 131 and x. So, according to the last equation, we have

solving for x we have the next steps:

next we multiply in the left side and then substract 526 to obtain

that is the answer.