Я думаю, вы хотите сделать это:
Program Calc;
var a,b,c,d: Real;
Begin
Write('a='); ReadLn(a);
Write('b='); ReadLn(b);
Write('c='); ReadLn(c);
if (a = 0) or (b = 0) or (c = 0) then
WriteLn('equation undetermined,S=R')
else
Begin
d := b * b - 4 * a * c;
if (d >= 0) then
Begin
WriteLn('x1=', (-b - sqrt(d)) / (2 * a):6:2 );
WriteLn('x2=', (-b + sqrt(d)) / (2 * a):6:2 );
end;
else
WriteLn('Equation has no real solutions');
end;
ReadLn;
End.