Write x^2 + 4x - 16 in the form (x+a)^2-b

Concentrate on the x^2+4x. Halve the 4 or any number that occupies that space. Equals 2 in this case. Put (x+2)^2 and that is your first term. Expand the brackets= x^2 + 4x + 4. Don't want the extra + 4 as that just results from the expansion of brackets, so minus the 4. Go back to the original equation (x^2 + 4x - 16) and there is a -16 on the end. Can put it all together into: (x^2 + 4x + 4) -4 - 16= (x^2 + 4x + 4)- 20. Final answer= (x+2)^2-20.