Remember the rules for order of operations:

Parenthesis, Exponents, Multiplication, Division, Addition, Subtraction (PEMDAS)

There aren't any Parentheses or Exponents,, so the next step is Multiplication and Division.

5 times 10 is 50, so the new equation is 50 + -18 = n + 55 

Next is Addition/Subtraction   positive 50 added to a negative 18 is positive 32, so we now have

32 = n+ 55.  We want n alone, so we subtract 55 from both sides to get

32-55 = n + 55 - 55.  Finishing up the Math, n = 32-55 = -23