agar garis y=3x+a menyinggung parabola f(x)=x^2-2x-8, nilai a sama dengan? tolong dibantu kak

f(x) = x² - 2x - 8
m = f'(x) = 2x - 2

y = 3x + a
m = 3

2x - 2 = 3
x = 5/2

f(5/2) = (5/2)² - 2(5/2) - 8
y = 25/4 - 20/4 - 8
y = 5/4 - 32/4
y = -27/4

-27/4 = 3(5/2) + a
-27/4 = 30/4 + a
a = -57/4