You need to use string formatting to show positive signs, or explicitly use +
in the string:
firstbinomial = (((b * -1) + math.sqrt((b ** 2) - (4 * a * c))) / (2 * a)) * -1
secondbinomial = (((b * -1) - math.sqrt((b ** 2) - (4 * a * c))) / (2 * a)) * -1
print "The binomials are: (x{:+.0f})(x{:+.0f})".format(firstbinomial, secondbinomial)
# prints "The binomials are: (x-3)(x+5)"
(which leaves the values as floats but formats without the decimal point), or
print "The binomials are: (x+{})(x+{})".format(firstbinomial, secondbinomial)
# prints "The binomials are: (x+-3)(x+5)"
The -
only shows because negative values are always printed with their sign.