I am trying to write a short script that finds when an equation stops decreasing and starts increasing again, keep getting error

%this script will find the minimum of a function/equation
clc
clear
%set variable equal to equation
t=-10:10;
%write for loop to loop our t values through our equation
for t=-10:10
    %equation 
 y=3.*t.^2+2.*t+40;
 %test to see if the next value in the loop is bigger than the last
 if abs(y(t))>abs(y(t-1))
     %if it is print result to command window
     fprintf('The minimum value occurs at t=%0.3f',(t-1))
 end
end

%this above is my script, whenever i run it i get this error 

% "Array indices must be positive integers or logical values.
%Error in Lab5Script3 (line 9)
 %if abs(y(t))>abs(y(t-1))"
 
 %is there any ideas you could give me to help solve this error?

[Rena Berman]

(Answers Dev) Restored edit

Answer 1

[Star Strider]

Define 'y' as an anonymous function:

y = @(t) 3.*t.^2+2.*t+40;

There are still a few problems. I will let you sort those.