Problem 1
One angle is 5x, so let's say the other angle is y. The two angles are complementary meaning they add to 90. So 5x+y = 90. Solve for y to get y = 90-5x or y = -5x+90.
The other angle, in terms of x, is 90-5x or -5x+90.
We don't have enough information to solve for x.
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Problem 2
If 2x is some even integer, then 2x+2 is the next even integer up.
For example, if 2x = 8, then 2x+2 = 8+2 = 10. Adding 1 goes from even to odd. Adding 2 goes from even to even.
Note that 2x+2 = 2(x+1) = 2*(integer) is in the form of any even integer.
This shows that both 2x and 2(x+1) are even.