Answer: infinitely many solutions
Work Shown
6x + 35 + 9x = 15(x+4) - 25
6x + 35 + 9x = 15x+15*4 - 25
15x + 35 = 15x+60 - 25
15x + 35 = 15x + 35
We get the same thing on both sides, so we get infinitely many solutions.
The solution set is "all real numbers". Replace x with any real number you want. Use PEMDAS to evaluate each side. The two sides will simplify down to the same number.
For example, let's try x = 2
6x + 35 + 9x = 15(x+4) - 25
6*2 + 35 + 9*2 = 15(2+4) - 25
12 + 35 + 18 = 15(6) - 25
65 = 90-25
65 = 65
Now try something like x = 10
6x + 35 + 9x = 15(x+4) - 25
6*10 + 35 + 9*10 = 15(10+4) - 25
60 + 35 + 90 = 15(14) - 25
185 = 210 - 25
185 = 185
I'll let you try other values.