I'll walk you through the problem. Let's begin by writing:
[MATH]y=x^2+2.6x+3.6[/MATH]
We want to group together the two terms containing \(x\):
[MATH]y=(x^2+2.6x)+3.6[/MATH]
Now, we look at the coefficient of the linear term (the term with \(x\) to an implied power of 1, the term \(2.6x\)). we want to divide the coefficient \(2.6\) by 2 to get \(1.3\) and then we want to square the result to get \(1.69\). So, we add this value inside the brackets, but, we have to subtract it on the outside so that in effect we are adding zero to the expression, so that its value is not changed.
[MATH]y=(x^2+2.6x+1.69)+3.6-1.69[/MATH]
Now, we can write this as:
[MATH]y=(x^2+2\cdot1.3x+1.3^2)+1.91[/MATH]
Notice the expression within the brackets is now a square:
[MATH]y=(x+1.3)^2+1.91[/MATH]
Do you follow me so far?