What are the 2 ways to solve a system of equations to algebra

What is the missing step in this proof ?

An energy drink company claims that its product increases students' memory levels. to support its claims, the company issues advertisements claiming that 8 out of 10 people (chosen randomly from across the country) who tried their product reported improved memory. the missing component in this study is a .

Look at this pattern ; 1,4,9, number 10000 belongs in this pattern . what’s the place of this number?