These are supplementary angles. Meaning that there sum is 180 degrees. To solve for x you can create a formula that adds them together and equals 180 degrees like so...
6x - 10 + 6x + 10 = 180
Now solve for x by combining like terms
12x + 0 = 180
12x = 180
Isolate x by dividing 12 to both sides
12x/12 = 180/12
x = 15
Hope this helped!