To use substitution, you solve one of the equations for one of the variables; then you substitute the expression for that variable into the other equation.
If we solve the first equation for v we get :
Now substitute `3/4u+2` in place of v in the second equation:
The solution to the system is `u=-2,v=1/2`
Checking we get `3u+8=3(-2)+8=2=4(1/2)=4v`