if f(x)=1-3x/2+x find an exact value of

f(-1/2)

As you typed it,

f(-1/2) = 1 -3(-1/2)/2 + (-1/2)
= 1 + 3/4 - 1/2
= 5/4

but you probably meant
f(x) = (1-3x)/(2+x)
f(-1/2) = (1 - 3(-1/2))/( 2 + (-1/2))
= (5/2)/(3/2)
= 5/3