



 
Hi Nick, First of all (2x + 5)(x + 5) = 2x^{2} + 15x + 25 so your final equation is
At this point you can either use the general quadratic formula or you can complete the square. Let's complete the square. 2x^{2} + 15x = 2(x^{2} + 15/2 x) so to complete the square you need to add (15/4)^{2} = 225/16 inside the parentheses since x^{2} + 15/2 x + 225/16 = (x + 15/4)^{2}. So here is my solution.
Thus x = 10/4 = 5/2 or x = 40/4. Since x is the length of a side of the rectangle it can't be negative so x = 5/2 cm. I hope this helps,  


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