Question 359618
First you have to pick a variable to eliminate. I would pick y because one the them is negative. So, then you need to get the y's to equal each other so you multiply the entire bottom equation by 3 (even the answer). Then you add the x's together and the constants together and divide by the number in front of x. To find x. Then plug the x into the original equation to find y. You find y by subtracting the x on both sides of the equal sign then by dividing by the number in front of y (if there is one).

3(5x+y=7)

  15x+3y=51
+7.4x-3y=7
-------------
22.4x      =58
----         ----- 
22.4          22.4

(I put 58/22.4 into my calculator and turned it into a fraction)
x=95/56

5(95/56)+y=7

475/56+y=7
              -y    -y

83/56=7-y
         -7      -7

725/56-7=y

83/56=-y
----   ---- 
-1          -1

-83/56=y