Remember PEMDAS (Parenthesis, Exponents, Multiplication, Divison, Addition and Subtraction)
Also note that Multiplication and Division needs to be done left to right and the same goes for Addition and Subtraction.
So in your problem, the first operation we need to do is division.
68 - (18/3) x 6
68 - 6 x 6
Now we do multiplication
68 - 6 x 6
68 - 36
Finally we do subtraction because that's the only operation we have left to do
68 - 36 = 32
So your answer should be 32.