In function notation, where f is the function, f(x)=3x-10. Using f(x) means you can write things like f(0) which means: what is the value of the function when x=0? Think of f for flexibility, because functions have greater flexibility than variables like y.
You would simply add the last two equations together. If v=2x+1 and t=4x-1, you can add them up. This would make 2x+1+4x-1. If you combine like terms, 1 and -1 cancel out and you are left with 6x. This means x can be anything.