Let x and y equal our two numbers, then add them and set them equal to 27
x+y=27
Pick a variable to represent your larger number, then make an equation for it
y=2x+6
Substitute your equation for that variable back into the original equation so you get everything in terms of one variable.
x+2x+6=27
Solve!
3x+6=27
3x=21
x=7
Plug x back into the equation:
7+y=21
y=14