When the question is this

Use the formula pH = - log [H+] to find the pH of fresh coffee whose hydrogen ion is 1.3 x 10^-5 moles per liter. Round to one decimal place.

How do I solve?
Please give detail.

use the log function on a scientific calculator, and take the log of the concentration of hydrogen ions, then use the negative as your answer

so -log([H+]) = -log(1.3*10^-5) = 4.886 = 4.9

[H+] means concentration of hydrogen ion, which is given (1.3x10^-5 moles per liter)
so take the -log[H+] = -log(1.3x10^-5) will give your pH of the coffee which is pH=4.89