A is x years old. In 4 years time his brother B will be 3 times as old as him. Find the age of B in 4 years time.
$x$ is A's age and let's call B's age $y$ at this point in time
Hint : In 4 years time A: is $x+4$ years old and B is $y+4$ years old
We know that B will be 3 times older than A in 4 years, aka $3*(x+4)=y+4$
Solving for $y$ : $y=3x+8$
In four years time B will be $y+4$ years old or aka $3x+8+4 = 3x+12$