Binary search is used to find a value in a sorted list. I am not sure why you would need a binary search algorithm to plot two equations as a graph ... unless of course you have been asked to find where the two intersect then it would make a bit of twisted sense because you could push the results of the equations into two arrays and then search for equality for every value of the first against the second (basically a very long winded and problematic approach to solving simultaneous equations).
As for not being allow to use Math.sqrt .. tough break. Your lecturer is obviously a bit of a sadist. Your best bet would be to look at the Babylonian_method
. It translates better to pseudo-code than the high-low method (which is better for manual computation).
Personally, to start this project I would clarify the question with your lecturer. Then as Sean4u stated, write a square root function and test it thoroughly. Next I would write a binary search function that accepts an array and a search value and returns the index of the position or -1.
Hope that helps.