so the piecewise is made out of two subfunctions.
when x ⩽ 3, f(x) is 2x - 9, namely when "x" is 3 or less, so it includes 3.
when "x" is more than 3, x > 3, f(x) is 4 - x, however, this subfunction doesn't include 3, just values greater than 3.
so we want f(3), when x = 3, and for that we have to use 2x - 9, therefore
2(3) - 9 ==> 6 - 9 ==> -3.