Question 151379
Let a the first number and b the second one

When the first of two numbers is added to twice the second the result is 21, 

then a+2b=21

when the second number is added to twice the first the result is 18.

then 2a+b =18

a+2b=21
2a+b =18
doing first eq by -2 we have

-2a+-4b=-42
2a+b=18

adding both eq

-3b=-24

b=8 and a+2(8)=21
then a+16=21
a=5

Solution: a=5 and b=8