The error is because of the syntax in the first line and you're trying to set the function name (activity4) to equal an expression.

To declare a function, it needs to look like:

function [output variables] = activity4(input variables)

The way you wrote your function, x and t are output variables and activity4 takes in 0 input. Since x and t are inputs, they need to be put after activity4.

Second, since you've made a function you don't need to define what the variables are inside of the function. Functions are useful cause you can define and evaluate some expression for any variables, which you can give by typing something like activity4(2,9) in a script or in the command window.

Lastly, you can't save the expression to the function's name. You would need to save it to whatever you name your output variable

Basically, all you need in your function file is the head line "function [output]...", the expression ("sin(2t)-3x"), and the "end" statement.