I don’t understand answer to this problem
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General functional notation (e.g. d(t) ) is not supported. If you wish to draw a function such as d(t) = 12t+25, replace d(t) with y (or any other unused variable).
If you need to find the value of d(t) for 't = 4', you need to plug the value:
d(4) = 12*4 + 5
d(4) = 48 + 5
d(4) = 53