1. Given an equation and some values of different quantities, we can substitute the values of those quantities into the equation.
\[x=4, \: y=1, \: z=2\]then
\[v=3z^2-2(x+y)=3 \times 2^2-2(4+1)=2\]
3. We can substitute equations into other equations. Let
\[c=b+d\]then we can substitute the second of these into the first.