to be good at ...

to be bad at ...

an instrument

a guitar

to play the guitar

a piano

to play the piano

to cook

to bake

a picture

to draw a picture

to paint

to listen

easy

difficult

for example

he's good at playing the piano

I'm bad at cooking

she's especially good at playing the guitar

they're good at many things

they're for example pretty good at math

I think math is difficult