Some of the steps in the derivation of the quadratic formula are shown.

step 4: startfraction negative 4 a c + b squared over 4 a endfraction = a ( x + startfraction b over 2 a endfraction) squared

step 5: (startfraction 1 over a endfraction) startfraction b squared minus 4 a c over 4 a endfraction = (startfraction 1 over a endfraction) a (x + startfraction b over 2 a endfraction) squared

step 6: startfraction b squared minus 4 a c over 4 a squared endfraction = ( x + startfraction b over 2 a endfraction) squared

step 7: startfraction plus or minus startroot b squared minus 4 a c endroot over 2 a endfraction = x + startfraction b over 2 a endfraction

which best explains why the expression plus or minus startroot b squared minus 4 a c endroot cannot be rewritten as b plus or minus startroot negative 4 a c endroot during the next step?