If you plot the function in the range from -20:20, you can see, that it crosses the x-axis around -1.0772 (and probably nowhere else). If you need to plot the function, here's a possible way:
x=-20:0.1:20; % '0.1' is an arbitrary increment dx
y=20*x.^3+25;
plot(x,y);
In the current case the dx=0.1, and for plotting the -1.0772 you need to make this dx<0.0001, however, the full value the matlab gives is -1.077217345015942 (which is also rounded), thus theoretically, you would need to set your dx<10^-15. Plotting your answer does not make sense. You could put a text over the plot with your answer:
text(0,.5*10^5,'-1.0772')
