I don't understand what I did wrong, please help​

Step-by-step explanation:

Lets start with the easiest part of this question - simply plugging in to get easier equations:

Something that now immedietly pops out is that many of the terms would cancel if we added them together. We get the following results by adding the two equations together:

Now lets look at the other bit of inromation the question gives you -- that the two points are horizontal tangents. Taking the derivative of the standard form of a cubic, we get:

Since the points are both horizontal tangents, y' will be equal to 0 at the points. Thus, plugging in we get:

We again see a simple subtracting opportunity that will cancel out two terms:

Now going back to the equation we got by adding the two equations that we got from simply plugging our points in:

Now that we have b and d, we can now plug them back into our step one and derivative equations in order to get a simple system:

Solving for a and c, we get:

Thus, finally, our answer is:

Attached is desmos, which you can use to check.

step-by-step explanation:

you can add the two numbers and divide the result by 2. or you can find the gap between the two numbers and add half of this to the smaller number.