exam Maths Puzzle Numerical Ability

please help me solve using the elimination method

3x - 7y = 16
5x - 7y = 36

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exam Other Question

Find the perfect square trinomial whose first two terms are q^2 + 3q.

My answer is

q^2 + 3q + (9/4)

I did the "complete the square method" (1/2 x 3)^2 and got (3/2)^2 and finally 9/4. Is this correct? You roll a dice. What is the probability the sum of the first roll is 7 and the sum of the second roll is 10?