well, we know the piece-wise function is
hmmm so what's f(-1), namely what is f(x) when x = -1?
well, when x = -1, hmm -1 is less than 0, so the subfunction that applies to that will be when "x" is less than 0, namely the part of 7x + 10, so we can say
f(-1) = 7(-1) + 10
f(-1) = 3
so hmm what's f(0) or namely, what's f(x) when x = 0?
well, what subfunction applies when x = 0? well, it has to be the subfunction with "x greater or equals to 0", x ⩾ 0, so then is 7x + 20
f(0) = 7(0) + 20
f(0) = 20.