To solve the system of equations:

u7 - 2u3 = 56
u6 - 2u2 = 28

We can start by solving the second equation for u6:

u6 = 2u2 + 28

Now we substitute this expression for u6 into the first equation:

u7 - 2u3 = 56
(2u2 + 28) - 2u3 = 56

Simplifying the equation:

2u2 + 28 - 2u3 = 56
2u2 - 2u3 = 28

Now we can divide both sides of the equation by 2:

u2 - u3 = 14

This equation relates u2 and u3, but we still need more information to solve for the variables.