Doreen and Raju had the same amount of money. Lihua and Raju had $3406 altogether. If the three of them had $5010 altogether, how much more money than Doreen did Lihua have?

D= Doreen

R= Raju
L= Lihua

D + R= 2x
-(L + R)= 3406
_______________

2x-3406= 5010

find x

then take x and that is what Doreen(D) and Raju (R) have individually. Thus each has x amount of money. D=R= x

Then since you have the ammount of money that Raju had (R) since it is x (number that you found before)
Use that and plug into

L+R= 5010

R= x

so L + x = 5010
and L = 5010- x

then after that (now you have the ammount that Lihua(L) had)
final equation is (to find how much more money than Doreen did Lihua have thus meaning that Lihua had more money of course):

L-D = __?

you have L and you have D
L= 5010 -x
D=R=x