To solve for something means to isolate that thing on the left side of the equation.
d = (1/2) a t^2
Swap sides,
(1/2) a t^2 = d
Multiply by 2, divide by t^2,
(2/t^2)*(1/2) a t^2 = (2/t^2) d
Clear fractions and reduce,
a = 2 d/t^2
and the equation is solved for a.