Sure, I can help you with that! Let's break it down step by step.
First, let's simplify the addition inside the parentheses: (11-15) = -4 and (15-18) = -3.
Next, let's calculate the square root of 216. The square root of 216 is 14.6969 (rounded to four decimal places).
Finally, let's substitute the values into the expression: -4 + -3 - 14.6969 = -21.6969.
So, the value of the expression is approximately -21.6969. = -7-6√6
so -7-6√6 is your answer :)