Hello,

It's ok for the beginning

There's just a problem for the limit of t.

When x tends to 7-, this means that x tends to 7, and x is <7.

So t=7-x indeed tends to 0, but is t<0 or >0 ?

--------> x<7 --> -x>-7 ---> 7-x>0

Therefore, t tends to 0, with t>0, which is written " "

Is it clear for you ?