Question 773210
Your description is ambiguous.  


Trying phrase by phrase,
Five times a number----------------- {{{5x}}}.
Decreased by 18--------------------- {{{5x-18}}}.
Minus 4 times the same number-------- {{{5x-18-4x}}}.
Is -36 -------------------------------{{{highlight(5x-18-4x=-36)}}}


Trying to guess at the phrasing meanings,
Five times a number-----------{{{5x}}}
Decreased by 18 minus 4 times same number--------{{{5x-(18-4x)}}}
Is -36 --------------------------------------{{{highlight(5x-(18-4x)=-36)}}}


Those two equations are not equivalent.  Their solutions are not the same.  The problem description needs to be more precise in order to tell which form is the one expected.