急!數學問题,幫下我啦,答小小都好 20點

1. The result of subtracting the triple of a number from 51 is 30.find the number.
solution:Let n be the number
51- ?n+?n = ?+?n
51 = ?+?n
51-? = ?n
? = ?n
n = ?
2.the result of subtracting 18 from the perimeter of a square is 26.find the lenght of each side of the square.
solution:Let l be the lenght of each side of the square
?l-? = ?
?l = ?
l = ?
3.if the sum of two consecutive even number is 86,find the smaller number.
solution:Let x be the smaller number
the larger number is ?
equation:?