Alison and flynn’s combined age is 40. given that flynn is 4 years older than alison, write an equation to find alison’s age.

Let A = Alison's age, let F = Flynn's age. We will use these to set up a system of equations. 

A + F = 40
A + 4 = F

We can rewrite that second equation by subtracting F from both sides and subtracting 4 from both sides. This gives you a new system of equations:

A + F = 40
A - F = - 4

Notice the Fs are going to cancel leaving us to solve for A when we combine like terms:

2A = 36
Divide both sides by 2 to isolate variable A.

A = 18

Now we plug this into either equation to solve for F. I choose the second equation. 

18 + 4 = F
22 = F

Alison is 18 years old and Flynn is 22 years old.