Calculating rms values is meaningful for repetitive (periodic) signals only.
The interval for integration is one period. Using symmetries the interval can be reduced to e.g. 1/2 period, 1/4 period or other fractions.
Your sketch doesn't show explicitly the periodicity. One may assume, I assume, that the signal repeats periodically, so the period would be ...(your's to find).
Using the symmetry of the signal you can reduce the interval to a smaller value.
By the way: while the text in your image is crisp, the sketch is rather blurry and veeery hard to read.