Question 831689
Let the speed of the slow one be x
Let the speed of the fast one  be y
Both travel and meet after 2 hours so we have the equation
2x + 2y = 56
Then you have the fast one is 3 times faster than the slow one, so you have
y = 3x =====>>>> 3x - y = 0
Then you solve the system of equation
*[invoke linear_substitution "x", "y", 2, 2, 56, 3, -1, 0 ]
y=21 so the speed of the faster one is 21mi/h
TA-DAH
:D