f(x+h)=V+k where x=3 and V=7. h is the “within” value for x and k is the “within” value of V.
So f(x+h)=2(x+h)+1=2x+2h+q=V+k. (We could also write f(x-h)=V-k, etc.)
So we put x=3, V=7 and k=0.54: 6+2h+1=7+0.54; 7+2h=7+0.54, 2h=0.54, h=0.27.
Therefore, if x is within 0.27 units of 3, then f(x) is within 0.54 units of 7.