7x + 3z = 45
7x -7x+ 3z = 45-7x
3z = 45-7x
3z/3 = 45-7x/3
z=15-2.33x
2x+7z=19
2x+7(15-2.33x)=19
2x+7(15) (7)(-2.33x)=19
2x+105-16.31x=19
105-14.31x=19
105-14.31x-105=19-105
-14.31x=-86
-14.31x/-14.31=-86/-14.31
x=6
z=15-2.33x
z=15-2.33(6)
z=15-13.98=1.02