Here's what 50% AM looks like: -
Now imagine the slowish sinewave shape (in red) is your signal so roughly when your signal bottoms out at about -0.033 "units", this has to correspond to the -70% level as implied in my picture. Yes I know my pictures shows +50% and -50% but you can see that there will also be a point that equals + and -70%.
Your signal has to fit in those limits exactly and, because AM can be regarded as simple 2 quadrant multiplication, there needs to be a DC offset applied to your signal to get it to fit as I've prescribed i.e. the "centre line" of your signal needs to be moved to the nominal level in my picture.
Note that the above sentence previously said "the average level of your signal needs to be moved,,,," BUT, because the signal is asymmetric about the average level this was misleading.
Now I have no idea what the graph in your question is actually shwoing in terms of volts because the axis aren't labelled and believe you me I'm not going to start plotting out the formula BUT, I can see that the graph does not bear much resemblance to the formula in terms of amplitude so you are on your own on this.