Question 100104
Let x=Mary's age now
And let y=her sister's age now
Then x-3=Mary's age three years ago

Now we are told that x-3=3(y-3)------------------eq1
and that:
x=2y-------------------------------------------------eq2

substitute x=2y into eq1 and we get:

2y-3=3(y-3)  get rid of parens

2y-3=3y-9  add three and subtract 3y from both sides

2y-3y-3+3=3y-3y-9+3  collect like terms

-y=-6  divide both sides by -1

y=6--------------------Mary's sister's age now

substitute into eq2
x=2*6 
x=12-----------------Mary's age now age now

CK

x-3=3(y-3)
12-3=3(6-3)
12-3=3*3
9=9
also
12=2*6
12=12


Hope this helps---ptaylor