Answer:
its -3
Explanation:
because the second line or blue one starts at y=0 and to get that you must plug in an answer that will give you an equvlant to y=x 
- so y=(x+k)+3 
 - then what plus or minus 3 gives you 3 the answer is -3 because -3+3 equals 0
 - so that of course means the finle euation is y=(x-3)+3
 - hope that helps buddy