Hi there! 
To solve this problem, we need to set up two equations and use the system of equations to solve.
Let x be the first number. 
Let y be the second number. 
(x + y) ÷ 2 = 34
x = 3y
Now, we can use substitution to solve.
(3y + y) ÷ 2 = 34
4y ÷ 2 = 34
2y = 34
y = 17
Now, we plug the value of y in the equation to solve for x. 
x = 3*17
x = 51
Hope this helps!