Surface Area of cuboid = 2 (lb + bh + hl)
According to the question :
length = x, width = x-5, height = 0.5x
250 = 2 ( x × (x-5) + (x-5) × 0.5x + 0.5x × x )
250 = 2 ( x² - 5x + 0.5x² - 2.5x + 0.5x²)
250 = 2 ( x² - 5x + x² - 2.5x )
250 = 2 ( 2x² - 7.5x )
250 = 4x² - 15 x
4x² - 15x -250 = 0