Question

Use the first equation to solve for an expression equal to y

Answer 1

Given the equations:

`\begin{gathered} x+y=70\text{ ---- equation 1} \\ x-y=30\text{ ----- equation 2} \end{gathered}`

To solve for an expression equal to y, using the first equation, we make y the subject of the formula in equation 1.

Thus, from equation 1,

`\begin{gathered} x+y=70 \\ \end{gathered}`

Add -x to both sides of the equation.

Thus,

`\begin{gathered} -x+x+y=-x+70 \\ \Rightarrow y=-x+70 \end{gathered}`

Hence, we have

`y=-x+70`

The third option is the correct answer.