I’m getting this error when trying to calculate the value of dW2.

```
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-84-a06d396e2b09> in <module>
1 parameters, cache, t_X, t_Y = backward_propagation_test_case()
2
----> 3 grads = backward_propagation(parameters, cache, t_X, t_Y)
4 print ("dW1 = "+ str(grads["dW1"]))
5 print ("db1 = "+ str(grads["db1"]))
<ipython-input-83-32e5e48c7f51> in backward_propagation(parameters, cache, X, Y)
47
48 dZ2 = A2 - Y
---> 49 dW2 = (1/m) * dZ2 * A1.T
50 db2 = (1/m) * np.sum(dZ2, axis=1, keepdims=True)
51 dZ1 = W2.T * (dZ2) * (1 - np.power(A1, 2))
ValueError: operands could not be broadcast together with shapes (1,3) (3,4)
```

The formula for which is:

Note all the following assumes I’m doing 1/m * XXXXXXX e.g. (1/m) * DZ2 * A1

Writing the calculation out as DZ2* transpose of A1 will give me an error. Running this as DZ2 * a1 and transpose the result will work, but it’ll be the wrong shape…

Which it’ll be

```
dW2 = [[ 0.00102716 0.00870451 0.00334799 -0.00358614]
[ 0.00034367 0.00454148 0.00061431 -0.00230807]
[-0.00058242 0.0044083 -0.00480396 -0.00433106]]
```

Where it should be

```
dW2 = [[ 0.00078841 0.01765429 -0.00084166 -0.01022527]]
```

This is wrong, but I don’t know how to change the code without deviating drastically from the formula which is dW[2] = 1/m*dz[2]*a[1].T

I don’t believe it’s my calculation of dZ2 as that is simply A2-Y

My labID is hqifnokw if you wanted to look.