When solving a linear system by substitution, how do you decide which variable to solve first?

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

Which shows the correct substitution of the values a,b and c from the equation 1=-2x+3x^2+1

The fence around a quadrilateral-shaped pasture is 3a^2+15a+9 long. three sides of the fence have the following lengths: 5a, 10a-2, a^2-7. what is the length of the fourth side of the fence?