Answer

I’ll show you a slightly different way.
I am assuming you know basic algebra.

See, 5b < -10
5b + 10 < -10 + 10 ( by adding 10 both sides )
5b + 10 < 0
5*( b + 2 ) < 0 ( Factorize out 5 )

Now, just think for a second. The two numbers “5” and
“(b +2) ” when multiplied, we require a negative ( i.e < 0 )
result. Since 5 is positive, we must have that ( b + 2 ) must be negative. Why? because by convention only a positive number multiplied by a negative number yields a negative number and vice versa.

Thus, ( b + 2 ) negative means that
b + 2 < 0
b + 2 -2 < 0 – 2 ( by substracting 2 both sides )
b < -2