You have the following expression:
(x + 3)(x + 5)
In order to simplify the previous expression, use the distribution property:
(x + 3)(x + 5) =
x(x) + x(5) + 3(x) + 3(5) =
x² + 5x + 3x + 15 = (simplify like terms)
x² + 8x + 15
Hence, the simplied expression is x² + 8x + 15