Explanation:
there is nothing special about this. no need to panic.
any arithmetic operation with functions means that we do these operations with the expressions of the functions. that's all.
(s.t)(x)
just means (s×t)(x) = (4x + 4)(5x) = 20x² + 20x
(s + t)(x) = (4x + 4) + (5x) = 9x + 4
(s - t)(x) = (4x + 4) - (5x) = -x + 4
so, (s - t)(1) = -1 + 4 = 3