Suppose you had f=6, g=2, h=3 and you were asked to find
f - g - h
You would do it by substituting the given values for the letters to get
6 - 2 - 3
And this can be simplified to
1
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The question at hand is basically the same. You are given values of f(x), g(x), h(x) and asked to find their signed sum.
f(x) - g(x) - h(x)
= (3x+5) - (4x²-2) - (x²-3x+1) . . . . substitute the given values
= 3x + 5 - 4x² + 2 - x² + 3x - 1 . . . use the distributive property to eliminate parentheses. A minus sign in front changes the sign of every term inside parentheses.
= x²(-4-1) +x(3+3) +(5+2-1)
= -5x² + 6x + 6 . . . . . . collect terms . . . . matches the first choice