How to find the maximum of a function that has 4 variables?

I have this question to solve for maths, I am not interested in getting the result just like that, I want to calculate it myself so that I will know how later on by myself.

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A particle is launched with velocity v from ground level at an angle to the horizontal. It is possible to show that its height above ground is given by the function

sy(t) = v*t*sin (r) - (1/2) g*t^2

where g is the downward acceleration due to gravity. For what value of t is sy maximized?

You should derive an expression in terms of v; and g.

t =

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So from what I can tell I need to derive the function in a certain way (way to go sherlock), something that I can do easily with a simple function with just one variable.

I am wondering how would I go about solving this and how do you "derive an expression in terms of v; and g" ?