Can someone help me with this math

Which equation and solution can be used to solve this problem? thirteen less than a number is sixteen. 13-n=16: add 13 to both sides. the answer is 29. n+13=16: subtract 13 from both sides. the answer is 3. n+16=13 subtract 13 from both sides. the answer is 3. n-13=16: add 13 to both sides. the answer is 29.

The equation x2 – 1x – 90 = 0 has solutions {a, b}. what is a + b?

Express the following as a function of a single angle. cos(60) cos(-20) - sin(60) sin(-20)