Question

y=4x-10 2x-3y=-10

Answer 1

2x-3y=-10

2x-3(4x-10)=-10 ... replace y with 4x - 10 (since y = 4x - 10)

2x-3*4x-3*(-10)=-10

2x-12x+30=-10

-10x+30=-10

I'll let you finish

Answer 2

Two equations, two unknowns. So this is two lines. Where lines intersect is the solution, a point, unless they are parallel. There many ways to solve this problem, but substitution as @jim_thompson5910 suggests is one straight-forward way that will always work. Another is to set both equations equal to y and equate. Another is to set each equation equal to y and add a constant multiple of one to the other in order to zero out one of the variables, leaving only one with an easy single equation and single unknown.

If you have any interest in solving using one of these other methodologies, let me know.