Algebra(1 of 3)
posted by Jon .
Solve the matrix equation 2x + 5y = 1
x + 3y = 0 for y.
A)1
B)3
C)3
D)1
I chose D
2x + 5y = 1
y = 5  2x
x + 3(5  2x)= 0
x + 15  6x = 0
7x = 15
x = 15/7
I know that's wrong I don't know what to do

Your very first step is incorrect. Try this:
x + 3y = 0
x + 5/2 y = 1/2
Now subtract the last eq. from the first.
y/2 = 1/2
y = 1 
your solution is "riddled" with errors
2x + 5y = 1 solved for y is y = (12x)/5
not y = 5  2x like you had.
Even if it had been right, x + 15  6x = 0
becomes
5x = 15 , not 7x=15 like you had
lets start over.
Why not solve the second equation for x? (look for the "single" variable)
x = 3y sub that back into the first
2(3y) + 5y = 1
y = 1
y = 1 looks like your D was the right guess. 
 15 x ² + 2x + 13 = 0