The number is 28.
The equation would be:
237 + 6(x) = 405
The problem states that 6 times the opposite of a number, however, since the number sign was not given, we do not know whether it is in a positive or negative form. So we leave it as only x.
237 + 6x = 405
6x = 405 - 237
6x = 168
6x / 6 = 168 / 6
x = 28
To check:
237 + 6 (28) = 405
237 + 168 = 405
405 = 405