Question

Write the quadratic equation whose roots are 2 and 1, and whose leading coefficient is 1

Not sure even how to start?

Answer 1

recall that if we have an equation factored equal to zero, then roots can be obtained by setting each of the factors equal to zero
e.g. if (x-6) is a factor, then x = 6 is a root

so reverse your thinking.
If the roots are 2 and 1, then the factors must have been (x-2) and (x-1)

so 1(x-2)(x-1) = 0
or if expanded:
x^2 - 3x + 2 = 0