3y^2 - 15 = 0?

3y² = 15
y² = 5
y = ± √5

In addition to the positive answer people gave, there is the negative answer, namely, the negative square root of 5.

y = sq rt of 5

And you want to solve this? Add 15 to both sides. Divide both sides by 3. Take the square root of both sides.

3y^2 - 15 = 0
3(y^2 - 5) = 0
y^2 - 5 = 0/3
y^2 = 5
y = √5
y = 2.23606797749979

y= square root of 5, or 2.2360679774997.

Hope this helps!