ezplot(y=t^3-9*t, [-4,4,-30,30])
 ezplot(y=t^3-9*t, [-4,4,-30,30])
         ↑
{Error: Incorrect use of '=' operator. To assign a value to a variable,
use '='. To compare values for equality, use '=='.
} 
ezplot('y=t^3-9*t', [-4,4,-30,30])
hold on
ezplot('y=t^3-(49/4)*t', [-4,4,-30,30])
ezplot('y=t^3-16*t', [-4,4,-30,30])
hold off
ezplot('y=1+cos(t)', [0,10,-2,4])