Let x = cups of 11% wheat, and y = cups of 63% wheat
He wants 12 cups in total, so we need:
x + y = 12
These 12 cups need to have 50% wheat, which means we are looking at 12*.5 = 6 cups of wheat in total, so our second equation is:
.11x + .63y = 6
We have two equations and two variables. So we can solve by using any means you want. I used substitution:
x = 12 - y
.11(12-y) + .63y = 6
1.32 -.11y + .63y = 6
.52y = 4.68
y = 9
Which means x = 3
Thus, 3 cups of 11% and 9 cups of 63%.