Question 1061388
We give representations:

let l be the age of Laura
let s be the age of Laura's son

According to the problem, Laura is four years less than twice her son's age

we write that as l = 2*s - 4 (we let this be equation 1)

It also says there that The sum of their ages is 119:

l + s = 119 (we let this be equation 2)

What we do now is to use equation one in equation two (substitution) 

l + s = 119 
(2*s - 4) + s= 119
3*s = 119 + 4
3*s = 123
s = 41

So, the son's age is 41. Now we solve for Laura's age using equation 1

l = 2*s - 4
l = 2*41 - 4
l = 82 - 4
l = 78

Laura's age is 78.

To check if we got it right,

l+s = 119

41 + 78 = 119

119 = 119