Hi syiro,
First you have to thoroughly read through and understand the material, so first you have to find the 3 equation.
first paragraph (in english) u gotta find the equation for the total number of houses sold of each kind equals to the capital
1: 3A+5B+6C=630
on the 2nd line of the 2nd paragraph (in english)
2: A+B+C=140
thrid paragraph, here it says the profit, so in order to get the profit of each house, you gotta take the selling price of each house minus off the construction costs so,
3:A(5.5-3) + B(9-3) + C(11-6) = 515
A+B+C=140
3A+5B+6C=630
A(5.5-3) + B(9-3) + C(11-6) = 515
so from these 3 equation, Im pretty sure you know how to do from that on, try it out 
Good Luck with that. Hope that helps.