Step-by-step explanation:
I am not sure what your problem is.
you don't know how to enter a formula into an Excel cell ? or you don't know how to combine the content of cells into one result in another cell ? or ... ?
but just answering literally this one-line-problem-definition
you click into an empty cell and type
=3*(50+40)/5
and press return.
and the cell will show you the result. 54