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First, as pointed out in the comments, you have the equation for a circle there.

Second, as an easy simplification, you might want to replace $\sqrt{25}$ with just a number.

Third, your figure consists of pieces of lines, so we would expect to see linear equations. I would recommend checking or figuring out how to calculate an area between two graphs. You will also have some piecewise defined functions, so knowing how to integrate those is necessary.

For a simpler task, where you only have one of these issues at once, you could halve the square so that you get a triangle, and calculate the area of that.