So if a number is divisible by 2, then the number is even. is this biconditional or is there a counterexample?

Biconditional means they can both be the hypothesis and the conclusion.

If a number is divisible by 2, then the number is even.  TRUE

If a number is even, then it is divisible by 2. TRUE

Since both statements are true, then it is a biconditional statement.

Biconditional

x = ±sqrt(10)

step-by-step explanation:

5 - 2x² = -15

subtract 5 from each side

5-5 - 2x² = -15-5

-2x^2   = -20

divide by -2 on each side

-2x^2   /-2= -20/-2

x^2 =10

take the square root on each side

sqrt(x^2 )=±sqrt(10)

x = ±sqrt(10)

check:

5 - 2x² = -15

+sqrt(10)

5 - 2(sqrt(10))² = -15

5 -2(10) = -15

5 -20 = -15

-15 = -15

-sqrt(10)

5 - 2(-sqrt(10))² = -15

5 -2(10) = -15

5 -20 = -15

-15 = -15