Hello, I have this signal in the picture and I need to turn it into frequency . How can I do this please I have no idea where to begin with . It's the second convo in the picture.

There isn't much to go on here. What are these graphs of? If the x axis represents time, then I guess you want to look at Fourier transforms.

I assume the first convolution was a square wave convolved with itself. In the frequency domain, convolution is equivalent to the multiplication of Fourier transforms. That means the operation does not add or subtact any new frequencies to the mix, it merely alters the relative amplitudes of frequency components that already exist within the transforms. In your examples, it is suppressing the upper harmonics in the spectrum of the original square wave, and then the triangle wave. So the fundamental frequency, which is what you are mostly observing in the second convolution, is the same as the fundamental frequency in the original square wave - given by the reciprocal of that wave's period.

These wiki entries may help:

https://en.wikipedia.org/wiki/Convolution

https://en.wikipedia.org/wiki/Triangle_wave