The main idea is let y be a subject and bring in the first equation and now it just have one letter in first equation, when you resolve the value of x, finally put the value of x into first or second equation then you can calculate what the value of y
The third choice is right. The constant is 5.
In the equation 4x + 5...
4x is a term.
4 in 4x is a coefficient.
x is a variable, or a letter that represents an unknown value.
5 is a constant.
There are 5 people who can be guards, 2 will be guards. Since each person has the same chance of becoming a guard, each person has a 2/5 chance of being a guard which makes it unlikely for Diana to become a guard.