數學function題 [急!!!]

s(x) = 2x + 2
t(x) = 5x

(a)
(s - t) (x)
= s(x) - t(x)
= (2x + 2) - 5x
= -3x + 2

(b)
(s ‧ t) (x)
= s(x)‧t(x)
= (2x + 2)(5x)
= 10x² + 10x

(c)
(s + t) (x)
= s(x) + t(x)
= (2x + 2) + 5x
= 7x + 2

(s + t) (-2)
= 7(-2) + 2
= -14 + 2
= -12