Chem: Ph, and structural formula for hydrocarbon?

Question

a few structural formulas i couldnt figure out

and need

3. 4-methyl-1-pentene

21. 2,3,4,5-tetramethylnonane

someone helped me with problem below, but my answers came out way wrong

143.pH of sea water is about 8.1

Calculate pOH, [OH-], and the [H+]

Answer 1

CH2 = CH CH2 CH (CH3) CH3

CH3CH (CH3) CH(CH3) CH (CH3) CH(CH3) CH2CH2CH2CH3

pOH = 14 - 8.1 = 5.9

[OH-] = 10^-5.9 = 1.3 x 10^-6 M

[H+] = 10^-14 / [OH-] = 8 x 10^-9 M