How do you solve `9x+5y=-21` and `-2x+y=11` using substitution?

`-2x + y = 11`.

When you are trying to find something that is lost always work backwards. PEMDAS backwards become PE SADM
(if you lose something in PE you are a sad member of humanity.
so the opposite of `-2x = + 2x`

Add `+ 2x` to both sides of the equation

`-2x + 2x + y = y + 2x`

`-2x +2x = 0`

leaving

`y = 2x + 11`

Now substitute `+2x +11` into the first equation for `y` giving

`9x + 5(+2x + 11) = -21`

Remember PE always comes first. Use the distributive property to multiply `5 xx (+2x)` and `5 xx 11`. This gives

`9x + 10 x + 55 = -21`

You are trying to find x which you somehow lost so you are a SAD M. Subtract `55` from both sides.

`+ 55 -55 = 0" "` and `" "-21 - 55 = -76`

leaving you with

`19 x = -76`

SA are gone so now you have to Divide. (D M) divide both sides by `19`.

`(19x)/19 = - 76/19`

`19/19 = 1" "` and `" "- 76/19 = -4`. so

`x = -4`

Substitute `-4` into the first equation

`y = 11 + 2(-4)`

`y= 11 + (-8)`

`y = 3`

If you remember that when you are trying to find something lost you are a SAD M and you won't be sad.