We would use the first equation to solve the second.
First, rewrite x+y=300 as x=300-y
Now we can substitute that for x in the second equation, allowing us to only have to solve for y.
300-y+3y=18
300-4y=18
-4y=-282
4y=282
Now, divide both sides by 4 to isolate x.
Y= 70.5
Now that we know the value of y, insert that in our first equation.
X=300-70.5
X=229.5