Look up a trig. identity for cos (a + b) by searching for "trigonometric identities.'
Then use a = 90 degrees and b = -10 degrees.
cos 90 degrees = zero, so the expression is just equal to the cos(-10 degrees).
How you should answer this problem depends on what references the you are allowed to use.
"Old school" trigonometry books had tables for all the trig function values of each individual degree.
I have such a book. I could give you the answer to four decimal places and the reference for the book.
Because of this, you might search for a "Trigonometry Table."
Provide me with the references you are allowed to use, and I will search for links to help you find a solution.
John