I will assume you need to find both the maximum value in b1 as well as its corresponding a1 value. All you need to do is use the max() function. The max() function will return both the maximum value, and the index position of the value. You can use the index to extract the corresponding a1.
Example follows:
a1=[1 2 3 4 5 6 7 8 9];
b1=[10 20 5 0 48 46 455 21 32];
[b1_max, index] = max(b1);
a1_max = a1(index);
You can also plot the maximum value as so:
plot(a1, b1, a1_max, b1_max, 'ro')
The 'ro' option tells matlab to plot (a1_max, b1_max) in red, and using an 'o' as marker.
Hope it helps!
