This is simple proportion, I guess. The value of y has increased by a factor 21/12 so x increases by the same factor 21/12*15=105/4=26.25. Or you could say that x is (5/4)y (15/12=5/4) so (5/4)*21=105/4=26.25.
If it's inverse proportion (x decreases as y increases) then we calculate 12/21*15=60/7=8 4/7 or 8.57 approx.
Take your pick!