book Maths Puzzle Numerical Ability

I dont quite understand ow to solve this problem.

Solve for a H=18t+1/2at(this last t is supposed to be squared)

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book Other Question

Complete the ordered pairs so that each is a solution for the given equation:
(show your work)

3x + 4y = 12 (0, ), ( ,3/4), ( , 0), (8/3, )
simplify the expression
-4(5c+7)-3c+13