Play dead math
Using the mean value theorem, when a>0 and b>0, a+b≧2√ab, and if and only if a=b, the minimum value is 2√ab, so when the formulas under the two radicals are equal, Y takes the minimum value, solves x=-6.5, and then 2√[(√x? +6x+73)(√x? -4x+8)] will do.