I'll just do the 3rd one, the hardest one:



To eliminate the x's:

The coefficients of x are 3 and 5.
The least common multiple of 3 and 5 is 15. We want to
make one of the coefficients of x 15 and the other one
-15.  To do that we multiply the first equation through
by 5 and the second one through by -3:



Add the two equations term by term
 





Go back to the original system:



To eliminate the y's:
The coefficients of y are 8 and 6.
The least common multiple of 8 and 6 is 24. We want to
make one of the coefficients of y 24 and the other one
-24.  To do that we multiply the first equation through
by 3 and the second one through by -4:



Add the two equations term by term
 






Edwin