Answer:
Step-by-step explanation:
[tex]x^2-15x-10=0\\\\x^2-2\cdot x\cdot\frac{15}2+(\frac{15}2)^2-(\frac{15}2)^2-10=0\\\\(x-\frac{15}2)^2-\frac{225}4-10=0[/tex]
So to complete the square we need to add (and subtract) the number [tex]\frac{225}4[/tex]