Question

find a and b so that f(x)=x^2+ax+b has a local min at (6,-5)

i found the derivative:
f'=2x+a

then i found a

0=2x+a
0=2(6)+a
0=12+a
a=-12

but i can't seem to get the correct value for b. i plugged -5 into the original function, but my book says the answer is b=31.