*Since I'm not sure if that "i" before the √15 , I'm going to assume it's not there. If another user knows what to do with the "i" then you should follow their answer.*
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Given the quadratic formula: x=−b±√b2−4ak2a we already know what the variable a and b are 4 and 7, respectively.
Now, we plug in the knowns into the formula, and we get: x=−7±√72−4(4)(k)2(4)
As a result, you get: x=−7±√49−16k8
Now, since we know that the number inside the square root is 15, to solve for k you can do this:
49-16k = 15
-16k = 15-49
-16k = -34
k = -34 / -16
k = 2.125
Therefore, k equals to 2.125
