Start by figuring out a good time range. Since you have cos(1256t-pi/4), the period is 
, so let's plot for two periods, i.e. time=0 to 0.01:
, so let's plot for two periods, i.e. time=0 to 0.01:t=0:.0005:0.01; %create vector of time values, with step size=0.0005
x=5*cos(1256*t-pi/4); %compute x(t)
plot(t,x,'r.-') %plot
Try it.
