# How do I do algebra subsitution?

## How do I do algebra substitution? Thanks

Jul 9, 2018

See below.

#### Explanation:

Suppose you are told that $x + y = 6$ and that $x = 2$. How do you figure out a value for y?

Substitute the value for x that you know (because of the 2nd equation you were given) for the x in the first equation.

$\text{ "" } 2$
$\text{ "" } \textcolor{red}{\downarrow}$
$\text{ "" "x + y = 6 color(red)" yields } 2 + y = 6$

That completes the substitution. But you do not have a value for y yet. That requires another of algebra's tricks: do something that helps to both sides.

We now have $2 + y = 6$. Subtract 2 from both sides.

$2 + y - 2 = 6 - 2$

$\text{ "" } y = 4$

I hope this helps,
Steve