Since you don't know how much Tanny paid, assign a name to that quantity.
The letter $x$ is a good choice. Then $x$ is the unknown value you want to
Take the way you would have calculated John's loss and Tanny's profit
if you had known the price Tanny paid, but use the symbol $x$ instead
of a known price.
Write out the calculations of how much John lost (as a percentage of
his cost, which was $540,000$) and how much
Tanny gained (as a percentage of his cost, which was $x$).
You will not be able to get a simple numerical percentage out of these
because of the unknown value $x$, but you will have two expressions
involving some known numbers, the unknown $x$, and some operations,
each representing a percentage.
The "is equal to" part of the problem tells you these two
percentages are equal. So you can put them on each side of an $=$ sign.
Now you have an equation, and the only unknown quantity in the equation
is $x$. You have to solve for $x$.
Hint: If the first step goes OK, on one side you'll have a fraction that is kind of ugly because it has $x$ in both the numerator and the denominator. You may want to recall rules such as cross-multiplying so you don't have an $x$ in a denominator.
Second hint: you'll probably end up wanting to solve a quadratic equation in order to finish.
It will be harder to follow these hints than if I had written out all
the formulas explicitly. What I'm trying to do here is to explain a little
of what would go through my head when looking at a problem like this,
and how I would get past the "what does this question even mean?" phase
and get something I could actually use some math to solve.