If you put the script and local function ga in the same file, then local functions should be placed at the end of the files as shown below. You can also put the local function in a separate file,
figure;
tmin=-3;tmax=8;N=100;
dt=(tmax-tmin)/N; t=tmin+dt*[0:N]';
g0=ga(t);g1=-3*ga(4-t);
subplot(2,1,1);p=plot(t,g0,'k');set(p,'LineWidth',2);grid;
ylabel('g(t)');
title('(a)-g(t)=-2,t<-1,2t,-13')
subplot(2,1,2);p=plot(t,g1,'k');set(p,'LineWidth',2);grid;
xlabel('t');ylabel('-3g(4-t');
function y=ga(t);
y = -2*(t<=-1)+2*t.*(-1<t&t<=1)+(3-t.^2).*(1<t&t<=3)-6*(t>3);
end
