Let x and y be the two numbers
Saying "The sum of two numbers is 27" means x+y = 27. The term "sum" is the result of adding. So the first equation is x+y = 27
The second equation is y = x+3 where we assume that y is the larger number of the pair.
So your system of equations will be
x+y = 27
y = x+3
Use substitution to solve this system.
x+y = 27
x+( y ) = 27
x + (x+3) = 27 ... notice how y has been replaced with x+3 (since y = x+3)
x+x+3 = 27
Solve for x and then use this to find y. I'll let you finish up.
If you need more help, then please let me know.