First, let's simplify the expression for Y:

Y = 8x² + 6x + √(-x² - 23x - 90)

Now, let's simplify the square root term:

√(-x² - 23x - 90) = √(-(x² + 23x + 90))

Now, factorize the quadratic term inside the square root:

√(-(x² + 23x + 90)) = √(-1)√((x + 18)(x + 5))

√(-(x² + 23x + 90)) = i√((x + 18)(x + 5))

Now, plug this back into the expression for Y:

Y = 8x² + 6x + i√((x + 18)(x + 5))

So, the simplified expression for Y is:

Y = 8x² + 6x + i√((x + 18)(x + 5))