ANSWER :
C. x^2 + 3x = 28
EXPLANATION :
From the problem, we have :
[tex]\begin{cases}2y=6x{} \\ x^2+y=28{}\end{cases}[/tex]
Express the first equation as y in terms of x :
[tex]\begin{gathered} 2y=6x \\ y=\frac{6x}{2} \\ \\ y=3x \end{gathered}[/tex]
Then substitute y = 3x to the second equation.
[tex]\begin{gathered} x^2+y=28 \\ x^2+3x=28 \end{gathered}[/tex]
