Will award brainliest please help

Choice C)  

We can say 4*sqrt(21) as optional formatting

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Work Shown:

Note how I factored each value in the square root such that we had one factor as a perfect square. That is useful to break up the root using the rule

I introduced the variable y to help show how the terms combine. This step is optional.

step-by-step explanation:

you can add the two numbers and divide the result by 2. or you can find the gap between the two numbers and add half of this to the smaller number.

not only in mathematics, scientific proof also are required in the other field of science. also maths proofs are of particular structure, containing steps and logics. so proving is like doing science, it is science. that's why it is important to prove. 

an inaccurate proof leads to errors, or stating results that are not accurate.  which wil mess the probelm up you dont want this