Use the FOIL method for (x - 5)(x-3)
FOIL = First, Outside, Inside, Last
x(x) = x²
x(-3) = -3x
-5(x) = -5x
-5(-3) = 15
x² - 3x - 5x + 15 = 0
simplify by combining like terms
x² - 3x - 5x + 15 = 0
x² - 8x + 15 = 0
x² - 8x + 15 = 0 is your answer
hope this helps