I need help with the ones marked red.
(See problem set in attached file).
The only formula you really have to remember is:
pH = -log [H+]
Everything else can be derived from that.
If the pH = 6.62, then:
6.62 = -log [H+]
or [H+] = antilog -6.62
Therefore, [H+] = 2.40x10-7 M (you forgot to round ...
It explains the relationship among pH, pOH, [H+], and [OH-].