Multiple 6 with numbers inside paranthesis:
12x - 66 + 15 = 21
add 66 fhen substrack 15 from both sides :
12x = 72 divide both sides with 12:
x = 6
While you cannot solve 946x+642y=911 for numerical values of x and y, you can indeed solve 946x+642y=911 first for x and then for y:
-911 - 642y
For x: 946x+642y=911 becomes 946x = 911 - 642y, or x = ------------------
946
911-946x
For y: 946x+642y=911 becomes 642y = 911-946x, or y = ---------------
642