Question 1084469
det a====b
===c====d
Here, the determinant is a--b
-------------------------c--d, where a is the 7 in 7x and b the 3 in 3y, c is 3 in 3x and d is 4 in 4y.
The determinant is found by multiplying ad and subtracting cd, and that will be the denominator.
det of 7==3
=====3==4
is 28-9=19
To solve for x, 
now put the 1 and 5 into the first column, and make a new determinant:
1====3
5====4
That product is 4-15=-11
so x=-11/19.
To get y, put the 1 and 5 into second column
7====1
3====5
so y=32/19
check -77/19+96/19=19/19=1
-33/19+128/19=95/19=5