I seem to recall that there are several definitions, but only very fuzzily :(

For sure you can apply Zadeh's extension principle to the maximum operation between real numbers. Then the membership function of the maximum M of two fuzzy sets A and B would be given by

M(x)=sup min(A(y),B(z))

where the supremum ranges over all pairs (y,z) whose maximum is x.

Doing that, in your example one obtains the fuzzy set which results from cutting the triangle by the y-axis and keeping only the positive part.