Question

2x + (20-y) x=6. y =17

Answer 1

Put the numbers in place of the letters first so 
2x= 2(6) 
20-y= 20-17 
so now solve 
2(6)=12+20=32-17=15
So your answer would be 15

Answer 2

(2x6) + (20-17)
12 + 3
15