Algebra help - substitutions in linear equations?

Question

3x + 6y = 15
substitute y with -2x + 3y = -3

please explain how to do that one ^ step by step. i mostly get how substitute the y when it starts out with a y (y = _x + _) but i need help with making everything equal y and putting it into the original equation.

if you could give me an answer for some of these too it would be great....

0.5x + 0.6y = 5.4
substitute y with -x + y = 9

15x -10y = -80
subs.'t y with 6x + 8y = -80

-3x + y = 10
subs.'t y with 7x + y = 20

x - 8y = -40
subs.'t y with -5x + 8y = 8

all answers whole #s

thanks soo much!

like, i know how to do this
y = -x + 3
y = x + 1

x + 1 = -x + 3
x = -x + 2
2x = 2
x = 1

y = x + 1
y = 1 + 1
y = 2

(1,2)

Answer 1

i'll do the first one for you:

3x+6y = 15

we will divide this equation by 3: x+2y = 5
and rearrange to give: x = 5-2y

now we have: -2x + 3y = -3

but we know x = 5-2y so

-2(5-2y)+3y = -3

expanding the brackets:

-10+4y+3y = -3

7y = 7
y = 1

Put this value of y back into the 1st equation: 3x+6(1)= 15

3x+6=15
3x = 9
x= 3

Answer 2

Multiply #a million by skill of two >>4x - 6y = 12 Subtract #2 from this >> x = 4 replace this into #a million >> 8 - 3y = 6 upload 3y to the two sides & deduct 8 from the two sides >> 8 - 6 = 3y i.e.3y = 2 Divide the two sides by skill of three >> y = 2/3

Answer 3

Lots of help at this website. You'll love the first one. Be sure to check them all out. Good luck.