Book Maths Puzzle Algebra

twenty-one is three times the difference between four times a number and five. find the number.

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Book Other Question

the difference between a number and fifteen is seven. find the number. A triangle has a perimeter of 197 cm. The sum of the smallest and medium sides is 49 more than the longest side. The difference between the longest and shortest sides is 22. Find the legth of all three sides.

HELP!

I know I have to solve by coming p with a system of equation, what I have is:

X=longest, Y=shortest, Z=medium

Y + Z = X + 49

X - 22 = Y

X + Y + Z = 197

Thank you for your time.