Question: How can I store a result directly to a function instead of a proc?

a := x -> x + 1;
b := x -> a(x) + x^2;
a := x -> x + 5;
b(x);
                            2        
                           x  + x + 5

However, I wish my b(x) to be x^2 + x + 1. How could I do it?

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